Eggs - we have them every morning, every day - they are soo nutrient rich and it’s easy for you too. Scrambled, poached or boiled are good.
Maybe try offering at different time? Omelette with cheddar seems to work like a charm for us. She will not touch eggs otherwise.
On days when my lo isn't hungry, I offer fruit and yoghurt and that usually goes down well. He does go through phases of not wanting breakfast but often makes up for it at other meals.
Eggs, toast, weetabix, cereal?
For breakfast i do a mixture and he loves it. Boiled eggs, small bits of toast, feta cheese, jam, olives, tomatos and milk