Bake for 25 minutes, tossing vegetables once after 15 minutes. Remove pan from oven and lower oven temperature to 350 degrees.
Place the skillet in the oven and cook for 25 minutes, until center of the frittata has set and top is golden brown.
Place the peppers and onions on a sheet pan. Drizzle with the olive oil, reserving one tablespoon for later. Toss with salt and pepper.
Sprinkle with Fontina cheese and bake another 3 minutes.
In a 10 inch ovenproof skillet, heat the remaining olive oil over medium heat. Add the roasted vegetables to the skillet. Pour the egg mixture over the vegetables and cook for 2 minutes without stirring.
Using a large bowls, whisk eggs, milk, and Parmesan.