3 tablespoons butter
1 small yellow onion, chopped
4 cups frozen shredded hash browns
1 pound bulk sausage, mild, hot -or- sage
2¼ cups whole milk
8 large eggs
1 teaspoon salt
¼ teaspoon black pepper
¼ teaspoon freshly grated nutmeg
2 tablespoons Dijon mustard
2 cups grated Cheddar
2 cups freshly grated Parmesan
8 cups cubed French or Italian bread, crusts removed
Preheat the oven to 350° F. Spray a deep 13 by 9-inch casserole dish with vegetable oil cooking spray. Melt the butter in a large frying pan. Add the onion and saute over medium-low heat until soft, about 5 minutes. Add the hash browns and break apart. Saute until soft, about 5 minutes.
In a second frying pan, saute the sausage, breaking apart large clumps. When the sausage is cooked through, remove it from pan.
In a large mixing bowl, combine the milk, eggs, salt, pepper, nutmeg and mustard, briskly to blend.
To assemble, spread the onions and hash browns evenly at the bottom of the greased dish. Place the bread cubes evenly on top of hash browns. With a slotted spoon distribute sausage as the third layer. Pour the milk and egg mixture over these layers. Add Parmesan as the next layer, while then adding the Cheddar.
Bake the casserole, uncovered for 45 to 50 minutes, until puffed and golden brown.