Step 1: Prepare the Ingredients

• Cook the sausage: In a large skillet, cook the Italian sausage over medium heat until browned and crumbled. Remove and drain on a paper towel. In the same skillet, sauté the chopped onion until soft, about 5 minutes. Add the spinach (if using) and cook until wilted. Set aside.
• Grease the baking dish: Lightly grease a 9×13-inch baking dish with butter or cooking spray.

Step 2: Layer the Bread and Fillings

• Layer the bread cubes: Arrange half of the bread cubes evenly in the greased baking dish.
• Add sausage and cheese: Sprinkle half of the cooked sausage, sautéed onions, spinach, and shredded Gruyère cheese over the bread.
• Add another layer: Repeat with another layer of bread cubes, sausage, onions, spinach, and cheese.

Step 3: Prepare the Egg Mixture

• Whisk the eggs and dairy: In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, salt, pepper, and nutmeg until well combined.
• Pour over the strata: Slowly pour the egg mixture over the bread and sausage layers, ensuring that all the bread is soaked with the egg mixture.

Step 4: Refrigerate Overnight

• Cover and chill: Cover the baking dish with plastic wrap and refrigerate for at least 4 hours or overnight. This allows the bread to absorb the egg mixture, resulting in a rich and custardy texture.

Step 5: Bake the Strata

• Preheat the oven: Preheat your oven to 350°F (175°C).
• Bake: Remove the strata from the refrigerator and allow it to sit at room temperature for about 30 minutes. Bake uncovered for 45-55 minutes, or until the top is golden brown and the center is set.
• Cool: Remove from the oven and let it cool for 5-10 minutes before serving.

Step 6: Serve the Breakfast Strata

• Garnish and serve: Garnish the strata with fresh parsley and serve warm. It pairs perfectly with a side of fruit or a light salad.