Preheat the oven to 375°F. In a large bowl, combine the ground beef, breakfast sausage, breadcrumbs, eggs, diced onion, salt, and pepper. Mix with your hands until evenly blended but do not overwork the meat.
Shape the mixture into a loaf and place it in a loaf pan. Bake for 45 minutes.
While the meatloaf bakes, stir together the tomato sauce, brown sugar, and mustard in a small saucepan. Heat over medium-low until the sugar dissolves and the sauce thickens slightly, then set aside.
In a separate bowl, mix the frozen hash browns, cheddar cheese, cream of mushroom soup, sour cream, and melted butter until everything is coated. Spread the mixture into a greased 9x13 baking dish.
After the meatloaf has baked 45 minutes, pour the tomato-brown sugar sauce over the top. Place the hash brown casserole in the oven on a separate rack and bake both dishes for another 25 minutes until the meatloaf reaches 160°F and the casserole is bubbly and golden on top.
Let the meatloaf rest 5 minutes before slicing. Serve slices with extra sauce spooned over and portions of the hash brown casserole on the side.