To begin, gather your ingredients so that you have everything close at hand on the counter or table. Spray a large casserole dish with baking spray.
Prepare Ingredients
Using a sharp knife and cutting board, slice bread into bite-size cubes. (Stale or 3-5 day-old-bread works best for this dish. I will often place week-old bread pieces in a plastic bag for the freezer for use in this recipe and also for using as croutons or stuffing.)
Brown the ground sausage in a hot skillet over medium heat. If necessary, drain off any excess fat.
With a box grater, shred the cheddar cheese. (I always recommend using block or chunk cheese over the grated store bought versions, despite the convenience.)
Mince the garlic.
Assemble the Breakfast Casserole
Place a thick layer of sourdough cubes on the bottom of the prepared casserole dish.
Sprinkle the browned sausage and shredded cheddar cheese over the sourdough cubes.
In a medium mixing bowl, whisk together the large eggs, whole milk, and heavy cream, minced garlic, dried minced onion, dried parsley, whole mustard, kosher salt and freshly ground black pepper.
Pour egg mixture over the sausage and cheese.
Cover and chill at least 8 hours or overnight. Bake uncovered in a preheated 350 degree oven for 50-55 minutes until lightly browned and set in center. Serves 8-10.