1 tablespoon EVOO
3/4 pound bulk spicy breakfast sausage
2 tablespoons butter
1 onion, finely chopped
4 cloves garlic, finely chopped
1 (9- or 10-ounce) box frozen chopped spinach, thawed and wrung dry in a kitchen towel
Salt and pepper
Freshly grated nutmeg
1/2 pound stale white or French bread, cubed (7 to 8 cups)
2 cups shredded caciocavallo or Parmigiano-Reggiano cheese
12 large eggs
2 cups milk or half-and-half
1/4 cup Dijon mustard
In a large skillet, heat the EVOO (1 turn of the pan) over medium-high heat. Cook the sausage, breaking it into crumbles as it browns. Transfer the sausage to a plate. Return the skillet to the heat, reduce the heat a little, and melt in the butter. Add the onion and garlic and cook to soften, 5 to 6 minutes. Pull the spinach into shreds as you add it and season with salt, pepper, and a few grates of nutmeg. Turn off the heat and stir in the sausage.
Arrange half of the bread in a 9 by 13-inch baking dish. Scatter half of the sausage-spinach mixture over the bread, top with half the cheese. Repeat the layering. Whisk together the eggs, milk, mustard, and salt and pepper. Pour the eggs evenly over everything.
Make-ahead: Cover and refrigerate for at least 24 hours for the bread to soak up the custard.
Night of: Return the strata to room temp for at least 30 minutes before you bake.
Preheat the oven to 350°F with a rack in the center of the oven.
Set the casserole on a baking sheet and bake uncovered for 1 hour. Let stand 10 minutes and serve.