Spinach, mushroom and fontina frittata

The past few weeks have been pretty hectic, so I've been preparing quick and simple food for dinner. I made up this dish based on what was in the fridge, so feel free to add and/or substitute ingredients. Also, all of my measurements are approximate.

Spinach, mushroom and fontina frittata
Serves 4

5 eggs
2 egg whites
1/2 cup caramelized onions
1/2 cup sauteed mushrooms
1 cup of frozen spinach, thawed and squeezed dry
1/2 cup of fontina cheese, cubed
1/4 cup of parmesan cheese, grated
A dash of nutmeg
salt and pepper to taste

Preheat the oven to 375 degrees. Heat a 10 inch nonstick ovenproof skillet on the stovetop. Whisk the eggs and egg whites together until thoroughly combined. Add the rest of the ingredients to the eggs. Pour the egg mixture into the skillet and cook until the edges have just started to set, about 3-5 minutes. Once the edges have set, transfer the skillet to the oven and bake for about 20 minutes, or until the eggs are set. Cut into wedges and serve.