Preheat your oven to 400 F. Grease a baking dish with cooking spray.
Whisk 7 eggs in a large bowl with milk of your choice and set aside. Add in salt and pepper and don't be afraid with these seasonings. Eggs are a blank slate.
Gather your leftover vegetables. Harder vegetables, like potatoes, broccoli and green beans, should be cooked before you add them to the frittata. Saute them in a pan with some oil and a pinch of salt until they're mostly cooked through - 10-20 minutes depending on the size and type of veggie.
For softer vegetables, like greens, zucchini, peppers and peas, you can leave them raw when you add them to the frittata OR you can give them a quick steam in the microwave. You can place these veggies in a microwave-safe bowl and partially cover them (I use a food container lid) and microwave for a minute.
For tomatoes and herbs (cilantro, basil, parsley, etc.) they can and should be added to the frittata raw.
Pour the whisked egg into a baking dish. Stir in cooked/prepped vegetables, salt and pepper to taste. Add the cheese, if using. Bake uncovered for 35-40 minutes, or until the top of the frittata is a deep golden color and it's firm to the touch.