Layer the cubed bread in the bottom of a 13” x 9” baking dish that has been sprayed with a cooking spray.
Ina large skillet melt the butter over medium heat, then add the chopped onion. Sauté until the onion is soft and translucent, about 8 minutes.
Add the hash brown potatoes, ½ teaspoon salt and ¼ teaspoon black pepper, stirring well to combine with the onion. Stir frequently to prevent the potatoes from sticking to the pan. Cook for about 8 to 10 minutes.Spread the potatoes and onions over the bread cubes.
Add the olive oil to the skillet and cook the sausage over medium heat, breaking it up as it cooks. Once the sausage is cooked through and no longer pink, about 6 to 8 minutes, spread it over the potatoes and onions.
In a large mixing bowl, whisk together the eggs, milk, grated cheese, 1 teaspoon kosher salt, ¼ teaspoon black pepper and nutmeg. Pour the egg mixture over the casserole. Cover and refrigerate at least 8 hours orovernight.
Prior to baking, pull the casserole our of the refrigerator and let it set at room temperature for about 30 minutes. Bake at 350 degrees for 55 to 60minutes or until the eggs are gently set and no longer “liquid” and the top is golden and puffed.
Let the casserole rest for about five minutes before serving.