Spray a 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 saute about 5 minutes. Spread the onions and hash browns at the bottom of the greased dish.
Arrange the bread cubes evenly on top of hash browns.
Using the same frying pan, saute the mushrooms for 2 minutes, then add the spinach leaves and saute until leaves are softened.
Layer the vegetables atop the cubed bread crumbs.
In a large mixing bowl, whip together the milk, eggs, salt, pepper, nutmeg and mustard, until well blended.
Pour the milk and egg mixture over the layers.
Add Parmesan as the next layer, and then sprinkle the Cheddar on top.
Bake the casserole uncovered for 35 minutes, until slightly golden brown.