I think this was originally in Gourmet. It is my go-to strata recipe, and can be used with any number of variations.

350F

• 1 (10-oz) package frozen spinach, thawed, or fresh equivalent, or chard, or a whole mess of fresh herbs
• 1 1/2 cups finely chopped onion (1 large)
• 3 tablespoons unsalted butter
• 1 teaspoon salt
• 1/2 teaspoon black pepper
• 1/4 teaspoon freshly grated nutmeg
• 8 cups cubed (1 inch) French or Italian bread (1/2 lb)
• 6 oz coarsely grated Gruyère (2 cups)
• 2 oz finely grated Parmigiano-Reggiano (1 cup)
• 2 3/4 cups 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 ceramic 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).  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.