Oven omelets -- also known as frittatas -- provide a convenient way to use up leftovers. You can use pretty much any vegetable you've got sitting around in your fridge waiting to grow fur. However, some people don't like them because they sometimes turn out flat and tough. With a little effort, you can make sure your omelets turn out light, fluffy and delicious.
Preheat your oven and cook your vegetables. Set your oven to 350 degrees F and set the rack at the center position. While your oven preheats, cook your vegetables. To do this, spray your pan with nonstick spray and place over medium heat. Add your vegetables to the pan and cook until slightly softened. If your veggies are already cooked, you can skip this step. Turn off the heat and set the skillet aside.
Beat the egg whites to stiff peaks using the mixer a large mixing bowl made of metal or glass. It's important not to use plastic because, as chef and cookbook author Alton Brown explains, plastic can adversely affect the creation of egg foams. You'll know you have stiff peaks when you can pull the beater out of the mix, turn it over, and a get a triangle-shaped puff of egg that does not collapse under gravity.
Break the whole eggs into a small bowl and beat with a fork until the white and yolk are thoroughly integrated. Using your large mixing spoon, gently mix the whole eggs into the egg whites. Brown recommends going easy with this, so that you don't break too many of the bubbles; this will keep your omelets light and fluffy. Once the whole eggs are mixed in, spoon the egg mixture over your cooked vegetables.
Place the skillet in the oven. Bake for 20 minutes, or until center is set. You can use a skewer or stick of spaghetti to check, or simply jiggle the pan a bit. The omelet is done when the center does not move relative to the rest of the eggs. Turn the omelet out onto a serving platter. If this is difficult, you can also use a spatula to slide the omelet onto a platter. Let cool for 10 minutes, slice and enjoy.
If you use high-water vegetables such as tomatoes, cook them until they've dried out a bit.
Don't beat the eggs until they start to harden up. That is over-coagulation and it renders the egg whites useless.