3 to 5 cup, mushrooms, vegetables or greens of your choice (or 3 cups leftover cooked vegetables or greens)
1 tablespoon extra-virgin olive oil
Garnish: chopped or torn fresh, leafy herbs (basil, parsley, cilantro, or dill)
Crack the eggs into a medium mixing bowl. Add your dairy of choice and the salt. Whisk just until the egg yolks and whites are blended. Whisk in all or half of the cheese (you can reserve the other half for topping the frittata before baking, if desired). Set the mixture aside.
Warm the olive oil in a 12″ cast iron skillet or oven-safe non-stick skillet until shimmering. Add the vegetables, starting with chopped onions or other dense vegetables. Cook for a few minutes, stirring occasionally, then add any softer vegetables such as zucchini. Cook until those vegetables are tender, then add any garlic or greens, and cook until fragrant or wilted. Season with salt, to taste.
Whisk the eggs once more and pour the mixture over the vegetables. Stir with a spatula briefly to combine and distribute the mixture evenly across the pan. If you reserved any cheese, sprinkle it on top of the frittata now.
Once the outside edge of the frittata turns lighter in colour (about 30 seconds to 1 minute), carefully cook until the eggs are puffed and appear cooked, and the centre of the frittata jiggles just a bit when you give it a gentle shimmy. Remove the frittata from the oven and place it on a cooling rack to cool. Garnish with herbs, slice with a sharp knife, and serve.