1. Heat oven to 350 degrees F. Coat a 10-inch nonstick skillet (with ovenproof handle) with cooking spray.
2. Heat butter in skillet over medium heat. Add onion and pepper and cook for 5 minutes, stirring occasionally.
3. In a large bowl, whisk together eggs, milk, 1/2 cup of the cheese, ham, salt and pepper. Pour mixture into skillet and stir to combine. Cook over medium-low heat for 5 minutes. Stir gently halfway through cooking.
4. Sprinkle remaining 1/4 cup of the cheese on top. Place skillet in oven; bake at 350 degrees F for 15 minutes.
5. Remove from oven. To serve, gently slide frittata onto serving plate and cut into 8 wedges.