Gain weight healthily

[TTPD]

Sorry, this has turned out longer than I thought!

I've always been underweight, since I was a young child, but I weighed myself this morning and I've dropped to 42kg. At my height this gives me a BMI of 15.6. I don't weigh myself very often, but the last time I did (maybe 6 months ago) I was 45kg and I think I had been around that for a few years (excluding while pregnant).

Generally I'm reasonably healthy - regular periods, not anaemic, not frequently ill etc, and I got pregnant with both my DDs at about 45kg. But I know this weight is not good. But I don't diet, I don't starve myself, I don't calorie count so don't know how many I eat - I couldn't even hazard a guess because I have literally no idea how many calories anything has.

On an average day I have
Porridge for breakfast - just plain oats (40g) and milk (150ml)
Something like sandwich/omelette/soup and bread for lunch, maybe an apple or banana as well.
Dinner is always homemade, and the last few nights we've had fish pie, a creamy chicken mushroom pasta thing, lasagne, chicken fried rice with veg. Sometimes something sweet for dessert as I do a bit of baking with DD1, at the moment we've got a chocolate sponge cake with mini eggs on top so I'll have a slice of that after dinner tonight. But that's maybe once a fortnight that we'll bake something to last a few days.
I don't tend to snack but only because I'm really not hungry. Not because I feel like I shouldn't, or because I'm restricting what I eat. If I'm hungry, I eat.

I do on average about 10,000 steps a day, but just from walking around, not "proper" exercise. I have very recently started doing some at home weights workouts to try and build some muscle.

How can I gain weight in a healthy way? I feel like I'd really be having to force more food in if I tried to significantly increase calories. But I don't want to be an unhealthy weight.

Have you ever had your thyroid checked because that seems like a reasonable amount of food unless your portions are really tiny and that’s a very low BMI.
look up body builder diets and base your diet on what they eat (but smaller portions obviously!) - lean proteins, nuts, oily fish, avocado, whole grain cereals and carbs, sweet potatoes, full fat dairy etc. I would also add some resistance training to help you build muscle.

I had a blood test recently because I've started having 10 day periods. I've just checked the nhs app - is "serum TSH" thyroid? Mine was 0.75, and the GP said all my results were fine. That's also how I know I'm not anaemic.

Portion wise I don't weigh anything except the porridge so not sure what size portions I have for dinner really because it gets split between the four of us. But for lunch if I have a sandwich it's two slices of bread, an omelette would be two eggs and cheese (I'd have some tomatoes with this as well), soup would be a regular tin plus two slices of bread. So I don't think I eat tiny portions.

I'll look into the bodybuilder stuff you've suggested. I can't eat nuts because DH has an anaphylactic allergy so we don't have them in the house, but the rest I can do.

Your lunch sounds quite small if you’re only having an omelette or soup and a couple of slices of bread might not be many calories. You could easily add hummus or soft cheese to your bread without making it much more filling. Or try turning your 40g of oats into a protein pancake for breakfast.
ive no idea about thyroid numbers I’m afraid but I know a number of people who’ve been told their blood tests are “normal” only to find they’re right at the bottom end of the normal range and hence still suffer from many of the symptoms of it being abnormal.