Start by cutting the day-old artisan bread into 1-inch cubes and set aside. In a large skillet over medium heat, cook the beef breakfast sausage until browned and fully cooked, about 5-7 minutes. Drain any excess fat, then add diced yellow onion and bell pepper to the skillet. Sauté for another 5 minutes until the vegetables are tender and fragrant, adding fresh spinach in the last minute if using.

In a spacious mixing bowl, whisk together 8 large eggs until well beaten. Gradually stir in 2 cups of whole milk and 1 cup of heavy cream, if desired. Then, add 2 tablespoons of Dijon mustard, 1 teaspoon salt, ½ teaspoon black pepper, ¼ teaspoon ground nutmeg, and a dash of Worcestershire sauce, combining until fully blended to create a custard-like mixture.

Grease a 9x13-inch baking dish with butter or non-stick spray. Layer half of the cubed bread evenly across the bottom of the dish. Next, add the cooked sausage, sautéed vegetables, and half of the sharp cheddar and Monterey Jack cheese. Repeat the layers with the remaining bread, sausage, vegetables, and cheeses.

Carefully pour the custard mixture over the layered casserole, ensuring all ingredients are evenly moistened. Press down gently with a spatula to eliminate any air pockets. Cover the dish tightly with plastic wrap and refrigerate for at least 8 hours or overnight.

When ready to bake, remove the casserole from the refrigerator to bring it to room temperature for about 30 minutes. Preheat your oven to 350°F (175°C). Cover the dish with aluminum foil and bake for 45 minutes.

After 45 minutes, carefully remove the foil and continue to bake for an additional 30-45 minutes until the top is golden brown and puffed, and a knife inserted in the center comes out clean.

Once baked, let the casserole rest for about 10-15 minutes to enhance the texture. If desired, garnish with fresh chopped chives or parsley. Slice into portions and serve alongside fruit salad or a bright green salad.