Spinach and Cheese Strata

INGREDIENTS

• 2 oz finely grated Parmigiano-Reggiano (1 cup)
• 2 tablespoons Dijon mustard
• 8 cups cubed (1 inch) French or Italian bread (1/2 lb)
• 1 1/2 cups finely chopped onion (1 large)
• 1 (10-oz) package frozen spinach, thawed
• 2 3/4 cups milk
• 1/4 teaspoon freshly grated nutmeg
• 9 large eggs
• 1 teaspoon salt
• 3 tablespoons unsalted butter
• 6 oz coarsely grated Gruyère (2 cups)
• 1/2 teaspoon black pepper