With my boys when I found they had limited eating, I started what to a degree I still do.
5 things on the plate... 1-2 always eat foods, 1-2 sometimes eat food & 1-2 never eat foods... so say with the dinner you had, I'd do 2 of those, then I might put say a small pile of grated cheese & 1/4 apple, (always eat) & some of the bacon pulled from the frittata (sometimes eat) & be 'ok' with the fact that they might only eat the apple & cheese. As the habit got more in play, they learnt they couldn't ask for more of a preferred food until they had eaten 3 (then 4) things from the plate... even now if I serve up something that DS2 turns his nose at I still do it in a way that he has 5 things on his plate & has to eat 4 of them before he gets more of something he does want... so say it is curry & rice, it is the meat, veg, sauce & rice (I put a small spoon full on his plate & then I might put 1/2 slice of bread... not enough to fill him up, so he will ask for more, but he knows he has to eat the "taste" before he can get a tin of spaghetti & eat that. or boil an egg (which he will do, with minimal help)
I know I was a limited eater as a child & my brother ate everything... as adults we are the reverse & being a researcher by profession, I read up on tastebuds etc & know that from 2-5 & older, children taste food much much more strongly than do adults, so they are sensitive to flavours, so I worked with that & honestly DS1 has taken off... he can eat curries that are hotter than I can & apart from cooked carrots & raw tomato, I can't think of much he doesn't eat... compared to the 2yo who ate 15 foods (total)... FWIW DS2 was a far better eater at 2yo & he is now where near as good at 9yo as DS1 was, so I don't think what he does now will be long term the issue.