Grease a 9x13 inch baking dish and set aside.

In a large skillet over medium heat, cook the breakfast sausage, breaking it up into crumbles, until browned and cooked through. Drain any excess fat.

In the same skillet, sauté the diced bell peppers and onions until softened, about 5 minutes. Remove from heat.

Spread half of the cubed bread evenly across the bottom of the prepared baking dish. Top with half of the cooked sausage, sautéed veggies, and shredded cheddar cheese. Repeat with the remaining bread, sausage, veggies, and cheese.

In a large bowl, whisk together the eggs, milk, sour cream, Dijon mustard, garlic powder, thyme, salt, pepper, and nutmeg until well combined.

Pour the egg mixture evenly over the bread, sausage, and veggies. Press down gently with a spatula to ensure the bread soaks up the liquid. Cover the dish tightly with plastic wrap and refrigerate overnight (or for at least 4 hours).

The next morning, preheat the oven to 350°F. Remove the casserole from the fridge and let it sit at room temperature while the oven preheats. Uncover and bake for 45-50 minutes, or until the top is golden brown and the center is set.

Allow the casserole to rest for 10 minutes before slicing and serving. Garnish with chopped fresh parsley, if desired.