In a frying pan on medium heat fry the onions and and peppers, add a little salt. Add the spinach and sauté until onions and peppers are caramelised. Turn of heat. Remove from pan and keep aside.
In a bowl, add eggs, milk, salt, and pepper. Whisk well. Add feta and vegetable mixture. Mix well.
In the same frying pan, put back on medium heat and add oil to the pan to ensure the omelette won’t stick. Add egg mixture and cook on stove top for 5 mins.
Make sure your pan is oven safe.
Place pan in the oven for 20 minutes.
Let cool for 5 minutes.
Notes
If you don't have frozen spinach, use 2 cups fresh spinach.