To be upset by PILS choice of celebration meal.

I think I would have just asked the staff to bring me the plate of vegetables without the gravy

Whilst I agree it was thoughtless of them, maybe they dont know as well as you think. I also think you may have been playing the martyr a bit, I'm sure a word in the staffs ear could have produced an alternative ie sides without gravy and a fruit juice starter; few restaurants would want you sitting there hungry.
Did you really have no idea of what you were eating until it was put in front of you?

I think generational differences may play a part. I am very diligent about checking this sort of stuff, partly as I seem to know so many people with children with allergies. My in-laws would be far more likely to think there was nothing to worry about with their relatives, (and my in-laws could plan a meal for their family without a single vegetarian involved), and forget individual ones in the planning of an event if not a big ticket ‘threat of death’ type one that scared them. Obviously that’s a generalisation but I don’t think most of my relatives - at 70+ - would engage a lot other than to think what sounded like the nicest meal.

For the restaurant not to do a verbal ‘no one with allergies, is that right?’ is odd though.

As a meal, it was thoughtless on your PIL’s side. However, did you ask the restaurant staff for an alternative? If you didn’t, you ABU.

YABU because you failed to ask the staff for an alternative.

There was no requirement for them to order on everyone's behalf though. Very strange choice imo

Bizarre all round (not you Op) to even arrange a meal with the same set menu for everyone. Even set menus usually have a few different choices/alternatives. I don't eat smoked salmon or lamb either. Not eating 2 things does make someone a fussy eater.

Also very poor service from the restaurant to not notice you weren't eating, or ask about allergies etc.

Could you not have quietly gone and asked the restaurant staff for an alternative and offered to pay yourself? Did you just sit there with the food in front of you.

I would imagine that the restaurant asked the hosts if any of their guests had allergies.

Bit crap, weirdly awful hosting. Unless you think it was deliberate I would probably just let it slide and be more conscious of checking ahead in the future.

You don’t sound fussy. But a set menu is never going to please everyone. Salmon is a genuine allergy and therefore you should have reminded them when you accepted the invite rather than relying on them to remember. DH has a shellfish allergy and every invite he accepts he always tells the host even if they’ve been told before just in case. Lamb is just bad luck, whatever they served chances are that 1 of the 20 wouldn’t have liked it. Unfortunately it was you. But I don’t understand why you sat there until pudding not eating. It sounds like the venue was a restaurant open to the public and serving others diners to order so why didn’t you just ask the staff to order a veggie option off the main menu and pay for it yourself?

It's a bit annoying yeah but sounds like they were just a bit thoughtless rather than particularly hostile. Planning something like that for 20 odd people can be pretty stressful and i don't think it's beyond the stretches of reality to accidentally forget what every person does and doesn't like. Tbh if I had allergies and was a bit if a fussy eater I'd have checked what was the menu so I could have made arrangements with the restaurant for food I could eat.

Unless there's some back story where they do this often I'd chalk it up to experience and move on.

But it wasn't your evening. They were thoughtless indeed but you could have mitigated this.

Yes it was a bit thoughtless. But why on earth would you just sit there and say nothing? Even asking for a new plate of just the sides without lamb/gravy would have been better than silent stewing and hunger.

If you aren’t prepared to advocate for yourself in such circumstances, is it any wonder that people forget your dietary preferences?

Well, it was thoughtless of them. However as soon as I knew what the main was I would have spoken to a member of staff and asked if there was anything they could provide for me to eat.
Having said that, I always ask what the menu is before confirming I'm going if I'm invited out for a set menu.

I'd try to assume the least worse of them, that they simply forgot, but I would probably still feel a bit hurt. It's not unreasonable to expect ILs you've known for decades to remember something like that, and the whole setup sounds rather odd. Salmon and lamb aren't the best choices, if they wanted something nearly everyone would enjoy. I understand not wanting to appear rude by speaking to the servers about an alternative, though that probably would've been better than missing almost the entire meal.

My MIL, who is lovely and to whom I do not ascribe any negative intentions here, ALWAYS forgets my food preferences. I hate creme eggs; she gets me one every year. Her beloved son hates Thorntons chocolate and for some reason she still buys them for him. I think she herself likes them and just forgets that others don't! So it might not be inconsiderate, it might just be the self-centredness of advancing age.

It’s mad and quite dangerous to not check on allergies before serving a set meal.

I don’t think you’re fussy. Being allergic to something isn’t being fussy. And you don’t like one thing out of hundreds which is lamb, the one meat they decided to go for. I also hate lamb. I eat any other meat but find lamb revolting.

it’s very strange they didn’t ask if anyone wanted the veggie option, I would be put out by that.

Why couldn’t you have said to the waiting staff on arrival that you’re vegetarian? I planned a big dinner for 30+ people this week. I had to put in pre orders but they still checked dietary requirements with each guest. At the very least they should check for allergies?! We had one too many vegetarians and the restaurant were very accommodating. You shouldn’t have been a martyr about it even though your ILs should have checked with people too.

@Shardlake63 I do wonder, how is your relationship with them in general?

On the face of it, it looks somewhat passive aggressive, IF it's not good.

When inviting my closest relatives to such a special event, I would, personally, consider all possible meal preferences before booking.

Anyone hosting an event like that should check dietary requirements. So basic.

And the restaurant should check when you sit down.

Did you "not want to make a fuss"? Because you should have said something to the staff, it's a restaurant, they can easily provide something else.

It says in the OP which surely everyone reads even if they don't read the later posts from the OP, that everything that came with the main course was smothered in lamb gravy so she couldn't eat any of it.

It's hardly a fuss or entitlement to want to not be served the only two things that she cannot eat. It makes her in-laws very poor hosts. If there is only one option for the starter and main course, you make sure that what you serve can be eaten by everyone you have invited.

I'd say nothing at all (no good will come of that) but next time they visit I would 100% serve something they hate/are allergic to.

Do the PIL have form for this kind of thing? Doing it once might be a sad oversight but I wonder if they plain don't like you and think picky eating is silly? I don't. I'd have eaten the starter but cannot even stand the smell of lamb.

If you live 200 miles away I guess you don’t eat as a family often - how sure are you they know you hate lamb? If it was my hubbys family event he would ask them to check I can eat something as I am a ceoliac.

It sounds like your in laws were thoughtless but I do think that anyone with allergies/dietary requirements/intense dislikes of certain foods needs to speak up for themselves. I agree with others that it's really odd for a venue not to check dietary requirements when catering for a large group.