DP just invited four people to dinner tonight and assumed I'd cook

You gush about how lovely it is and how nice for him to cook for a change and you're so glad he's got into cooking so you don't have to be the sole cook anymore.

Chill. If they come last minute they may be happy there's something to eat. Let him make the food. Ask if you can help with the beans.
My ex's sister and husband once came over. He asked her if she could help me with the food. Of course! She cooks fabulous meals. She's Indonesian. He and her husband were busy with the bbq. She asks me: What can I do? Ehm throw this ready to eat bag of salad in this bowl while I put the potato things in the fryer. She had to laugh so hard.

I will admit I skim read the OP and thought "Don't eat his miserable bean stew" was an amazing metaphor.

OK, a few observations here:

Yes it was a shit thing to do to you and you need to have a conversation about how you both handled that.

Nevertheless, one of those moments where you pull together as a couple and one of you zooms round the house tidying and plumping, finding wine etc and the other throws some food together.

They are usually the best nights anyway.

Going forward I'd have a special store of really last minute easy to throw together cupboard recipes like crab linguine, garlic bread etc where you just need to run to the local coop for a few herbs, or grab a bought curry from Cook and make a biryani or something.
Or just buy something from Cook or other cook from frozen lifesaver.

I’d be going out! Even if it was to sit in my car reading a book somewhere random. This is so fucking rude and he won’t learn how unacceptable it is it you just go along with it

What's on at the Cinema? I'd be going out and having popcorn for dinner whilst watching something on the big screen.

I'd say to DP that you're sorry but you'd completely forgotten you've arranged to meet a friend this evening. Text FF girlfriend and say you're out but that you'd love to have them over another time soon. Then go out.

Go out, @Anthologist

He’s the problem. He’s hugely disrespectful. And he has obviously used your good nature many times before.

I'd most certainly be out for the evening!

Cross post. Any half way decent pub in the middle where you can all meet instead of your house?

But still take the puss out of flakey DP

Go out. Cinema and a meal out, call a friend if you prefer company.
Your DP is obviously entitled to invite pals round but to expect you to cook at no-notice???? My DH knows he'd be ordering a takeaway under those circumstances. That's so not fair to drop that on you. He's cooking, so fair enough, but I'd go out and make bloody sure the dishes weren't left to me.
What an inconsiderate thing to do.

Well maybe he should be 🤷‍♂️

Have dinner with everyone, make sure everyone knows he cooked, join in with everyone laughing about how revolting his stew is, let him learn a lesson about last minute plans and humiliating himself. Be a hero and have some cheese and beans on toast at the ready.

Just order in, open some wine and take the piss out of DP who is hence forth known as the Flakey one. I suspect all the others will understand

Graciously accept his invite, tell your guests how pleased / surprised you were when DP said he was hosting/ cooking a last minute dinner party and let him get on with it. If it's a shit show don't say anything 'except, lovely to see you all' and 'It's my turn to cook next time!'

How often does he whip up impromptu dinners for your friends?

PotteringAlong has it right.

I'd be furious/mortified. I love it when I get a night to myself.

Yeah I'd go out too.

He is being very unreasonable and it's going to go badly.

Definitely go out.
Leave him to play host.

I would be furious if my husband did that and I think its incredibly telling he just expected you to cook for two extra (or i guess 4 extra) people with no notice

I’d definitely go out. If you’re in touch with the girlfriend, text her and say “Oops, DH just mentioned he’s invited you guys round tonight but as I thought he was coming to yours I’ve already got plans so I won’t see you. Have a nice evening and I’ll catch up with you next time!”

I’d either go out or tell him to call back and say he was a bit hasty, you have no food in and shall you all get a takeaway instead.

Going out would be my preferred option though.

You are definitely not being unreasonable and well done for not cooking.

I'd make it crystal clear that your dp has done the cooking

Even if I were the best cook in Britain, I would have gone ballistic at any partner simply expecting me to cater at short notice for friends THEY had invited round.

I'm including any of my partners thus far in this declaration - thankfully, my female partner is a much better cook than I am but we also ALWAYS make decisions about inviting people TOGETHER.

children?

I'd take a book and go out to the local Chinese restaurant.

It's great he's cooking. But he doesn't get to organise your evening - and if you were looking forward to being alone, then do so. I wouldn't fancy either his cooking or a meal with FF, french gf, and two other fussy eaters to be honest.

The key thing was 'you are invited if you want to be involved'. Just don't.