The best way is to offer plenty of variety, model eating alongside them - making sure you’re eating a healthy variety too, and take the pressure off so be busy focusing on your own meal without trying to force/ persuade/ pressure them to eat themselves.
You can boost/ add nutrition to meals so add berries, grated apple or grated veg (carrot and courgette work well) to porridge, with nut butter or ground almonds or milled seeds etc.
Mix breakfast up, sometimes give weetabox or another fortified cereal for an iron boost, sometimes give scrambled eggs and avocado or eggy bread with spinach. Another day pancakes - you can make the baby ones with a mashed banana, two eggs and 3 spoons of porridge oats - they look rank but my dd laps them up with yoghurt and berries.
Add things like grated cheese or full fat yoghurt or extra veg or milled seeds or nutritional yeast or eggs or mashed beans/ lentils to stuff like pasta, mashed potato, muffins etc. Use a mix of different grains eg pasta, bread, rice, couscous etc.
Stock up on frozen veg and throw a bit of that on the plate for every meal you can - even if they refuse at first.
My dd goes through phases where she will only eat fruit, yoghurt and pasta but I just keep offering all the other stuff and eventually she picks it up again.
Stuff that goes down really well here are - broccoli/ cauliflower cheese, really thick homemade veggie soups with bread dipped in, hummus and pita, cheese and spinach eggy bread, mashed sweet potato with butter beans and grated cheese mashed in, any savoury muffins like courgette and chickpea ones from the what mummy makes cookery books - they’re honestly very easy to make. Also omelettes with extra veg in and some peas on the side.
My dd is still very hit and miss with meat (I don’t eat it so maybe I haven’t exposed her to it enough) and she won’t eat a sandwich at all. Other mums I know have had a lot of success making pinwheels (sheet of ready made puff pastry, spread whatever on eg tomato purée, cheese, finely chopped or grated veg, roll up slice and bake) but mine won’t touch them - might be worth a try for you too?
I make extra of anything we are having that is freezable and freeze in portions and if I ever get chance to make muffins or whatever I do that then freeze a load, so there’s always something in the freezer we can grab for her if for whatever reason what we’re having isn’t suitable (today I felt really sick and overstuffed after Christmas so just had cereal but got some cauliflower cheese and mash out of the freezer for dd for example).