Spinach and Cheese Strata

NEED:
1 (10-oz) package frozen spinach, thawed
1 1/2 cups finely chopped onion (1 large)
3 tablespoons unsalted butter
1 teaspoon salt
1/2 teaspoon black pepper
1/4 teaspoon ground nutmeg
8 cups cubed (1 inch) French or Italian bread (1/2 lb)
6 oz coarsely grated Gruyere (2 cups) ( you can use any hard yellow cheese you wish)
2 oz finely grated Parmigiana-Reggiano ( 1 cup)
2 3/4 cups of milk
9 large eggs
2 tablespoons Dijon mustard

Squeeze handfuls of spinach to remove as much liquid as possible, then finely chop.
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 third of bread cubes in a buttered 3-quart gratin dish or other shallow baking dish and top evenly with one third of spinach mixture. Sprinkle with one third of each cheese. Repeat layering twice (ending with cheeses).
Whisk together milk, eggs, mustard, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl and pour evenly over strata. Chill strata, covered with plastic wrap, at least 8 hours (for bread to absorb custard).
Oven at 350 degrees. Bake uncovered for 45 to 55 minutes--until puffy and golden brown.
Let stand 5 minutes before serving.

Strata can be chilled up to 1 day. Let stand at room temperature 30 minutes before baking.

Enjoy!