Heat oil and butter in your oven-safe skillet over a medium flame. Add the onion and sauté until translucent, about 4 minutes.
Meanwhile preheat the broiler on low. Whisk the eggs, garlic, salt, pepper, milk, and parmesan cheese in a bowl. Then pour into skillet and stir to distribute the onions evenly and add the ham evenly over the top. Cover and cook until the egg is almost set, about 2 minutes. Then uncover and place the skillet under the broiler about 7 inches from the heat source, until the top is set and golden brown and fluffy, about 4 minutes. Cut into slices, garnish with fresh herbs, and serve.
Frittatas are a great way to use up leftovers. Try this gluten free frittata with ham or let your imagination take over and create your own combination.
Frittatas pack well so you can bring a few slices on a picnic or bring a slice to work for lunch.