Heat oven to 350ºF. Whisk together eggs, milk, Cheddar and Parmesan cheese; season with salt and pepper as desired. Set aside.
Melt butter over medium-high heat, in 10-inch oven-proof skillet with 2-inch sides. Add sausage and bell pepper; cook and stir until pepper has softened slightly, about 3 minutes.
Stir in green onions; remove pan from heat. Pour egg mixture into skillet; stir to combine.
Bake 30-40 minutes until eggs are set and knife inserted near center comes out clean.