This is one of those classic old recipes which I got from Gourmet. The original called for frozen spinach but I have always used fresh. I brought this for our Teacher Appreciation Brunch today where it was much appreciated. This is a foolproof recipe and there are never any leftovers.
3 – 4 shallots or 2 spring onions, chopped fine
1 big bag baby spinach (10 – 12 oz.) , roughly chopped
3 tablespoons butter
1 teaspoon salt
1/2 teaspoon black pepper
1/4 teaspoon nutmeg
8 cups cubed (1 inch) French or Italian bread (1/2 lb) (I used whole wheat)
6 oz coarsely grated Gruyère (2 cups)
2 oz finely grated Parmigiano-Reggiano (1 cup)
2 3/4 cups milk
9 large eggs
2 heaping tablespoons Dijon mustard
Cook shallots/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. Add spinach in handfuls; as it wilts, add more, until all of it is wilted, then remove from heat.
Spread one third of bread cubes in a buttered 3-quart gratin dish or 9 x 13 inch glass 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).
Preheat oven to 350°F. Let strata stand at room temperature 30 minutes.
Bake strata, uncovered, in middle of oven until puffed, golden brown, and cooked through, 45 to 55 minutes. Let stand 5 minutes before serving. It is fine to serve at room temperature.