Preheat oven to 425 F.
Generously grease a large oven-proof skillet such as cast iron. (If you don't have oven-proof skillet, cook in any skillet to begin with and then transfer to greased baking dish to put into the oven.)
In the skillet, saute scallions or onions and garlic in a generous amount of olive oil for 2 or 3 minutes.
Stir in vegetables and cook till nearly tender. (Tomatoes alone will only take 2 or 3 minutes.)
Stir in the spinach and cook till wilted (probably less than a minute).
Stir in eggs and cheese and a little pepper. (Note, it's plenty salty because of the cheese.)
Stop stirring and let the mixture cook until the edges are set, about 2 minutes.
Put in oven and cook 8 to 13 more minutes, till top of frittata is just set. (Toothpick comes out clean.)