Yield: 8 to 10 servings.
1 loaf brioche bread
1 pound ground breakfast sausage, cooked and drained
1 tablespoon unsalted butter
2 cups onion, chopped
3 cups fresh baby spinach
2 cups Gruyère cheese, grated
12 large eggs
1 ½ cups half-and-half
1 teaspoon Dijon mustard
1 teaspoon salt
½ teaspoon freshly ground black pepper
Prepare a 9 x 13-inch casserole dish with cooking spray. Tear brioche into large chunks and place in dish, overlapping to cover the bottom. Sprinkle cooked sausage on top of bread and distribute evenly.
In a skillet, melt the butter on medium heat and add chopped onions. Cook onions for about 5 minutes or until soft. Add spinach to pan and cook until spinach is just wilted. Pour cooked onions and spinach over top of sausage. Top with grated cheese.
In a bowl, whisk together the eggs and half-and-half. Add mustard, salt, and pepper. Pour the egg mixture over the bread in the pan. Cover with plastic wrap and refrigerate overnight.
The next morning, remove casserole from fridge, preheat oven to 375°, and place rack in middle position. Remove plastic wrap and cover baking dish with foil. Bake, covered, for 30 minutes. Remove foil and bake until top is slightly puffy and center is set, about 30 minutes more.
Let cool for about 10 minutes. Serve immediately.