In a large bowl, whisk together the eggs, milk, ground mustard and pepper.
5 large eggs, 1½ cups whole milk, ¾ teaspoon dry ground mustard, ¼ teaspoon black pepper
Add the hash browns, meat, and 1 cup of the cheese. Set remaining ¼ cup of cheese aside for the topping. Stir until thoroughly combined.
12 oz breakfast sausage links, 15 ounces frozen hash brown potatoes, 1¼ cup sharp cheddar cheese
Pour into pan. Cover with foil. Bake covered 1 hour.
Remove from oven, remove foil, top with the remaining ¼ cup of cheese, place back into oven uncovered for 10 minutes.
Once the top is lightly browned and the eggs aren't wet in the middle anymore, remove from oven. Let the casserole sit for about 10 minutes. Cut and serve.
Store leftovers in airtight container in the refrigerator for up to two days.
Notes
You can prep the casserole the night before (unbaked), put it in the refrigerator overnight, and bake it in the morning. It's best to take it out of the refrigerator and let it set at room temperature for 30 to 60 minutes before baking.If you prefer, use bacon or ham in place of the sausage.