1 (10-ounce) package frozen chopped spinach, thawed
1 1/2 cups finely chopped onion (1 large)
3 Tablespoons unsalted butter
1 teaspoon salt
1/2 teaspoon black pepper
1/2 teaspoon freshly grated nutmeg
8 cups cubed (1 inch) French or Italian bread (1/2 pound)
2 cups coarsely grated Gruyere (about 6 ounces)
1/2 cup finely grated Parmigiano-Reggiano (about 1 1/2 ounces)
1 cup cooked breakfast sausage, ham, or shredded meat (optional)
2 1/2 cups milk
9 large eggs
3 tablespoons Dijon mustard

Squeeze handfuls of spinach to remove as much liquid as possible.

Cook onion in butter in a large heavy skillet over moderate heat, stirring until soft, 4 to 5 minutes. Add 1/2 teaspoon salt, 1/4 teaspoon pepper, and nutmeg and cook, stirring 1 minute. Stir in spinach, then remove from heat.

Spread one half of bread cubes in a buttered 3-quart gratin dish or other shallow ceramic baking dish and top evenly with one half of spinach mixture. Sprinkle with half of each cheese. Repeat layering, ending with cheese.

Whisk together milk, eggs, mustard, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl and pour evenly over each side. Chill strata, covered with plastic wrap, at least 8 hours.

Preheat oven to 350 degrees F. Let strata stand at room temperature 30 minutes. Bake strata, uncovered, in middle of oven until puffed, golden brown and cooked through, 50 - 60 minutes. Let stand 10 minutes before serving.