Preheat oven to 375 degrees F.
Wash and cut all the vegetables.
In a mixing bowl crack the eggs and whisk with the salt, pepper, and milk. Set aside.
In an oven safe saute pan heat the olive oil and add chopped onion, saute until it is starting to get soft. Add the garlic and cook for a few more seconds.
Next add the tomatoes and cook for 2-3 minutes or until the slightly soften. Stir in the spinach.
Next pour the egg mixture over the vegetables in the pan.
Top with mozzarella cheese, let the eggs set a bit by baking them on the stove top for a few more minutes.
Then transfer the frittata to the oven in the pan and bake for 15-20 more minutes or until the egg is set. It will depend on how big your saute pan is and how deep the eggs were.
It is done when you can put a knife in the middle and it comes out clean.
Serve with basil leaves if you would like!
