Transfer the skillet to the oven, and bake for 15-20 minutes or until the eggs have just set.
Notes
You can use heavy cream or half-and-half in place of the milk for a richer flavor.
You can use butter in place of the olive oil.
Use whatever kind of onion you prefer, such as white, sweet, yellow, or red.
You can swap the ham for a different meat, such as crumbled sausage or bacon.
You can swap the cheese for a different kind, such as gruyere, American, Colby, gouda, or mozzarella.
Feel free to add some veggies, such as diced bell peppers, mushrooms, broccoli florets, tomatoes, or spinach.
You can also add more herbs or seasonings, such as basil, tarragon, or Italian seasoning.
Shred a block of cheddar cheese to avoid the added starch in pre-shredded cheddar cheese. Finely shredded also melts better without getting a gritty texture.
Always cook your meat and veggies in the pan before adding your eggs. Leaving them slightly underdone before popping them in the oven will also prevent your frittata from getting soggy.
Storage: Store ham and cheese frittata in an airtight container in the refrigerator for up to 3 days or in the freezer for up to 2 months.