12 oz baby spinach leaves
1/2 lb thick-cut bacon or pancetta, cut into 1/4″ slices
1 1/2 cups onion, finely chopped
3 tablespoons unsalted butter
1 teaspoon sea salt 
1/2 teaspoon freshly cracked pepper
1/4 teaspoon freshly grated nutmeg
1/2 lb 1-inch-cubed French bread (with good crust)
2 cups Gruyère cheese, coarsely grated
1 cup parmigiano reggiano, finely grated
2 3/4 cups whole milk
9 large eggs
2 tablespoons Dijon mustard
In a large nonstick pot, sauté the spinach over medium heat until it wilts. (If not using a nonstick pot, add a tablespoon of olive oil first.) Put spinach in a colander and let any excess liquid drain in the sink while you prepare the other ingredients.
In a medium nonstick pan, cook the bacon over medium heat until it browns. Drain the bacon on a paper towel-lined plate to cool.
Wipe out the pan and cook the onion in butter over moderate heat, stirring until soft, 4 to 5 minutes. Add 1/2 teaspoon salt, 1/4 teaspoon pepper, and nutmeg, and cook for 1 minute. If the spinach is still very wet, squeeze out any excess liquid with your hands. Stir spinach into the onion mixture and remove from heat.
In a large shallow baking dish, spread out one-third of the bread cubes and top evenly with one-third of the spinach mixture. Sprinkle with one-third of each cheese. Repeat the layers 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.
Preheat oven to 350 degrees. Let strata stand at room temperature 30 minutes.
Bake strata uncovered in the middle of the oven until golden brown and cooked through, 45 to 55 minutes. Let stand 5 minutes before serving. Serve with hot sauce (optional).
- If you want to save a little money, substitute the Gruyère with white cheddar.
- This is a great recipe for a group brunch, so prepare it the night before and let it sit overnight.