Brown the sausage in a large skillet and crumble into small pieces. Remove from the pan and drain reserving around 1 tbsp of fat in the pan. Set sausage aside.
Add the diced peppers and onion to the pan and cook in the reserved fat (or in 1 Tbsp of olive oil or butter) over medium high heat for around 3-4 minutes stirring occasionally until the onion is translucent.
Add the eggs, milk, and cream to a large bowl and whisk to combine. Season with salt and pepper.
Add hashbrowns to a baking dish along with the sausage, peppers, and onions and one cup of cheese then stir to combine and spread to an even layer.
Pour egg mixture into the baking dish, distributing it evenly over top of the potatoes and vegetables, then top with the remaining cheese.
To bake immediately - Bake at 375°F for 45-55 minutes until the edges are lightly browned and the eggs are set, with a just slight jiggle in the middle. Remove and let cool for 10 minutes before serving.
To make ahead - Cover the uncooked casserole and refrigerate overnight, up to 2 days. Remove the casserole from the refrigerator 15-20 minutes before baking.Bake at 375°F for around 55-65 minutes until the edges are lightly browned and the eggs are set, with a just slight jiggle in the middle. Remove and let cool for 10 minutes before serving.
Notes
Make Ahead - Casserole can be made through step 5 and refrigerated for up to 2 days before baking. If refrigerating remove the casserole from the fridge for 15-20 minutes prior to cooking. You may need an additional 10 minutes of cook time as well.Storage and Reheating - Store leftovers in the refrigerator for up to 4 days. Rehat large portions in a 350°F oven until warmed through. Individual portions can be reheated in the microwave.