Preheat oven to 350°.
Spray a 9x13 inch pan with cooking spray.
Combine the eggs, sour cream, milk, cheese and salt & 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 as it cooks.
Drain the drippings of the sausage from the pan.
Add the sausage to the egg mixture.
Add 1 tablespoon of butter to the skillet that the sausage was cooked in, sauté the green, red bell peppers and onions in the skillet for about 2-3 minutes.
Add to the bowl with the eggs and stir everything to combine.
Pour mixture into greased 9x13 inch pan and bake for 35-50 minutes or until the edges are set and the center is just barely jiggly.
Chef's notes: Leftover egg casserole can be stored in the fridge and enjoyed within 3-4 days. Reheat in the microwave. This casserole may be made a day ahead of time. Prepare as directed up until baking. Cover the casserole and store it unbaked in the refrigerator, overnight or up to 1 day. To freeze, prepare up to baking. Cover well with plastic wrap and then tin foil. Can be frozen for 2-3 months. Thaw overnight in the refrigerator, then let it come to room temperature before baking. You can add any veggies you like.