Spinach and Cheese Overnight Strata

Grease 3-quart gratin dish or 13 x 9-inch baking dish. Melt butter in large skillet over medium heat. Add onion; cook, stirring frequently, for 4 to 5 minutes or until onion is soft. Stir in two-thirds of crushed bouillon, 1/2 teaspoon pepper and nutmeg. Cook for 1 minute. Stir in spinach; remove from heat. Spread half of bread into prepared dish. Top with half of spinach mixture and then half of Gruyère cheese and half of Parmesan cheese. Combine milk, eggs, mustard, remaining crushed bouillon and remaining pepper in large bowl. Pour evenly over bread mixture. Cover with plastic wrap; refrigerate for at least 8 hours or overnight. Preheat oven to 350 °F (175°C). Let strata stand at room temperature for 30 minutes; uncover. Bake for 45 to 50 minutes or until puffed and golden brown. Let stand for 5 minutes before serving.