Preheat oven to 350.Spray a 9×13 inch pan with cooking spray.I used my butter wrappers to grease the pan.
Combine the eggs, sour cream, milk, cheese, salt, and pepper in a large bowl.Mix on low speed with electric mixers, just until combined.
Heat a large skillet over medium heat.Add sausage and cook until browned, breaking it into small pieces with a wooden spoon as it cooks.Drain most of the grease and add the sausage to the bowl with the egg mixture.
Add the bell peppers and onion to the same skillet the sausage was cooked in and saute for 2 to 3 minutes.Add to the bowl with the eggs and stir everything to combine.
Pour mixture into greased 9×13 inch pan and bake for 35 to 50 minutes or until the edges are set and the center is just barely jiggly.
Leftover egg casserole can be stored in the fridge and enjoyed within 3 to 4 days.Leftovers are delicious reheated in the microwave.