Preheat your oven to 375 degrees Fahrenheit and grease a 9x13 inch baking dish.
Press the thawed hash browns into the bottom of the dish to form an even crust.
Bake the potato crust alone for 15 minutes to slightly crisp the edges.
Brown the breakfast sausage and diced onion in a skillet over medium heat for 8 minutes, then drain the grease.
Spread the cooked sausage mixture evenly over the par-baked potato crust.
Sprinkle 1.5 cups of the cheddar cheese over the meat.
Whisk the eggs, milk, dry mustard, salt, and pepper in a large bowl until smooth.
Pour the egg mixture slowly over the layers in the baking dish.
Top with the remaining half cup of cheese and bake uncovered for 40 to 45 minutes until set.