we flew to America last year with 2 food restricted dds, and are off to Australia this year.
you can look up what airlines will carry, as some foods (like ones in sauces) are restricted.
My dds are gluten and dairy free.
we were able to book a meal for dd1 (can't rmember what the permutation was, as there is no gluten and dairy free option) but it was salmon and rice salad (dd1 was 3, and is autistic, so she didn't go anywhere near it!)
we took loads of stff with us, together with a letter form the doctor saying that dd1 had food issues (due to autism, and due to intolerances)
we will be doing the same again when we fly to Oz this summer.
Lakeland do a really good thermos flask, which keeps things hot (well, warm enough) for up to 7 hours, so that gets you through a good part of the first half. last year (11 hour flight) I took fishfingers and nuggets for the first meal, and home made fish pie for the second. we got through security ok by explaining and offering to taste.
take along a LOT of nutritious filling snacks too. we take Larabars,, but no good for you as contain nuts.
cereal is always an option - you can take along your own rice/soya/oat milk (or whatever you use) as long as you decant into 100ml bottles, although again we took 200ml cartons last year as dd1 would only drink one type of juice formt he carton. Again, we explained, and they took them away and screened them, but we got through ok. you can take as many 100ml bottles as will fit inot your clear plastic bags, and then multiply that by how many people flying... again, this is what I am planning for dd1 this summer.
we are flying ia Honk Kong, with a 4 hour stop, i think. we didn't want to stop for a couple of days precisely due to the food issues - would rather plan for the flight times (23 hours in total, so really only need 3 meals plus snacks) and then get to Australia and sort out our food form there, rather than having to run around finding food at stopovers.
will have a think, and see if I can remember what else I did/took