Whisk the eggs, milk and salt together in a small bowl until well combined. Set aside.
Heat 2 tablespoons olive oil in a 10-inch cast-iron or non-stick skillet over medium heat. If the onions and other vegetables are raw add them to the skillet with a pinch of salt. Stirring occasionally until they are tender, but not browned. If you're using raw spinach, add it next and sauté just until it is wilted.
Sprinkle the cheese on top and let it start to melt. Pour in the egg mixture and tilt the pan to ensure the eggs settle evenly over all the vegetables. Cook for a minute of two until you see the eggs at the edges of the pan beginning to set.
Remove from heat and place in the oven. Bake for about 10 to 15 minutes, or until the eggs are set. To check, push a table knife into the center of the frittata. If raw egg runs into the cut, bake for another few minutes; if the eggs are set, pull the frittata from the oven. For a browned, crispy top, run the frittata under the broiler for a minute or two at the end of the cooking time.
Cool in the pan for 5 minutes, then slice into wedges and serve.
Notes
What can you put in a frittata? I’m so glad you asked!! These are just ideas, you’re the cook, get creative!
Roasted Red Pepper, Broccoli, Onions/shallots, Mushrooms, Potatoes, Cherry Tomatoes (or chopped tomatoes), Fresh Spinach, fresh herbs like chives, tarragon, basil.