Beat the eggs and season with salt and pepper.
Cut onion, bell pepper, cherry tomatoes, and spinach into bite-sized pieces.
Sauté onion and bell pepper in a pan with olive oil.
Add all the vegetables and stir-fry together.
Pour the beaten eggs into the pan and spread them evenly.
Cover with a lid and cook slowly over low heat.
When the frittata is fully cooked, transfer to a plate.