Preheat oven to 350℉ (175℃) and lightly grease a 7x11 or 9x9 baking dish.
In a medium skillet over medium-high heat, brown the sausage until fully cooked through, breaking it into small pieces as it cooks. Remove from heat and drain off any grease as needed. Set aside.
In a medium mixing bowl, whisk together 6 large eggs, ½ cup milk, 1 teaspoon kosher salt, and ½ teaspoon black pepper until well combined.
Layer 16 ounces defrosted hash browns into the greased baking dish, followed with the cooked sausage. Sprinkle 2 cups sharp cheddar cheese over top the sausage and hashbrowns. Pour the egg mixture over the layered casserole. Use a spoon to push the ingredients down to ensure everything is covered with the egg mixture.
Bake for 40-45 minutes or until eggs are set and the casserole is slightly browned. Let rest for 5-10 minutes prior to serving.
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Notes
Hash Browns: Refrigerated or defrosted, frozen cubed or shredded hash browns work well. For added peppers and onions, use frozen or homemade potatoes O'Brien. Cheese Options: I love sharp cheddar, but Colby jack cheese, Swiss cheese, or pepper jack cheese are great options. For a creamy melt, grate the cheese yourself rather than using bagged shredded cheese. Sausage: Feel free to replace the sausage with cooked, chopped bacon or cubed ham. For the sausage, use bulk, not breakfast or Italian sausage unless desiring the flavor that comes from sweet breakfast sausage or spicy Italian sausage. Eggs: Feel free to add an additional 2 eggs to the custard if you like a really thick layer of eggs--just increase the bake time by 10 minutes. To make this casserole the night before serving: Assemble the casserole up to baking. Cover the casserole tightly with plastic wrap or airtight lid and refrigerate overnight. When ready to bake, remove the casserole from the fridge and let it sit on the counter for 20 minutes while the oven preheats. Remove the plastic wrap and bake at 350℉ for 55-60 minutes. Double the Casserole: Double the ingredients and bake as directed in a 9x13 pan. Pan Size: Be sure your pan is a 9x9 or 11x7, if using an 8x8 pan you may need to increase bake time by 10 minutes. Storage: Store leftover hashbrown breakfast casserole in an airtight container in the refrigerator for up to 4 days.