01 -
Preheat the oven to 350°F.
02 -
Press a piece of parchment paper down into a 9-inch cake pan, 9-inch springform pan, or 8-inch square baking pan, crumpling it to tuck it down in the corners. The parchment should completely line the pan, which will make the frittata easy to remove after cooking. Spray the parchment paper lightly with nonstick cooking spray.
03 -
In a mixing bowl, whisk together the eggs, milk, ½ teaspoon salt, and ½ teaspoon black pepper until evenly combined.
04 -
Heat a 10-inch nonstick pan over medium heat. Add the butter and let it melt. Add the vegetables and remaining ½ teaspoon of salt to the skillet. Cook until the vegetables are tender and heated through, 4 to 6 minutes if using raw vegetables or just 1 minute if using pre-cooked. Add the greens and cook until wilted.
05 -
Pour the beaten eggs into the prepared baking pan. Add the cooked vegetables and cheese, then stir gently, folding some of the vegetables and cheese into the center of the frittata while leaving some on top.
06 -
Bake the frittata in the oven until the center is just set (it should still jiggle a little when you shake the pan), about 30 to 35 minutes.
07 -
Let the frittata rest for 5 minutes at room temperature before slicing and serving.