But unsweetened (Greek) yoghurt with some oats/"granola?" and chopped fruit (like strawberries) is amazing.
Energy in energy out, Adult intake for average activity level is like 2k kCal or 8k kJ or so. I usually think like 1500/6000 or so since I wfh.
Let's do American so take your 1500 kCal, maybe leave keep 800 for dinner, 500 for lunch, 200 for breakfast (and snack during the day). So just make sure your breakfast is 150-200kCal.
Apparently it's 90kCal for a large egg (78g), so you could have 2 maybe with a dash of sauce of some kind for breakfast just fine.
But otherwise you could have anything: https://travel.earth/breakfast-around-the-world/ just ensure food type/portion size is controlled and balanced for vitamins etc as well. Soups (Asia and other parts) look pretty good too so I may try adding that in myself to my routine.
Just eat regular food, during breakfast time. (Although eggs are fine)
See The McLibel case, 1997 (1) particularly the statement "You could eat a roll of Sellotape as part of a balanced diet" (2)