In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add 1 pound of pork sausage, breaking it apart with a spatula. Cook for about 5-7 minutes until golden brown. Remove and set aside.

In the same skillet, add 1 medium chopped onion and 8 ounces of sliced cremini mushrooms. Sauté for 5-7 minutes until translucent. Add 5 ounces of fresh spinach, stirring until wilted, about 2-3 minutes.

Grease a 9x13-inch baking dish. Layer half of 1 pound of cubed sturdy white bread, followed by half of the cooked sausage, half of the sautéed vegetables, and half of 8 ounces of shredded Gruyère cheese. Repeat layers.

In a large mixing bowl, whisk together 8 large eggs, 2.5 cups of whole milk, and 1 cup of heavy cream until smooth. Stir in 1 tablespoon of Dijon mustard, 0.25 teaspoon of nutmeg, 1 teaspoon of salt, and 0.5 teaspoon of black pepper.

Pour the egg mixture over the layered ingredients. Gently press down on the bread to help it soak. Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Preheat oven to 350°F (175°C). Remove casserole from refrigerator and let sit at room temperature for 30 minutes before baking. Bake uncovered for 50-60 minutes until puffed and golden brown.

Let the casserole rest for 10 minutes before slicing. Serve warm.