Preheat your oven to 350°F (177°C) and lightly grease a 9 x 13" casserole dish.
Place the thawed hash brown potatoes into a colander and gently squeeze to remove excess moisture. Let them sit in the colander while you prepare the remaining ingredients. (Frozen hash brown potatoes can be thawed in the microwave if you are short on time.) See notes for using fresh potatoes.
In non-stick skillet, cook the breakfast sausage, peppers and onions over medium to medium-high heat for 7-10 minutes. The meat should be browned and fully cooked and vegetables softened. Remove the skillet from the heat and set aside to cool down.
In a large mixing bowl, whisk together the eggs, sour cream and half and half. Add the shredded cheese, hash brown potatoes, sausage mixture, salt, pepper and stir to combine.
Pour the mixture into the casserole dish and bake for 50-60 minutes or until the edges are set, lightly browned and the center is slightly jiggly. The center should register at least 160 °F (71.1 °C) using a digital food thermometer.
Remove the casserole from the oven and let it rest for 15 minutes in order to firm up before slicing.