Preheat the oven to 350 degrees F (180 degrees C). Butter a 9x13-inch baking dish.
Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Set aside to cool slightly.
Add butter to a griddle over medium-high heat; spread potatoes in an even layer, and cook until golden, 5 to 6 minutes per side. Add potatoes to the bottom of the prepared baking dish.
Whisk eggs and cream together in a large bowl. Whisk in dry mustard and season with salt and pepper. Stir sausage and 8 ounces Cheddar cheese into eggs. Pour mixture over potatoes, and smooth out the top. Cover the dish tightly with foil or an oven-safe lid.
Bake in the preheated oven until eggs are nearly set, 30 to 40 minutes. Remove from the oven, remove foil, and scatter remaining cheese over the top.
Return casserole to the oven and bake until eggs are no longer jiggly, about 10 minutes. Turn the oven up to Broil, and broil until top is browned and crispy, a few minutes more.
Serve immediately with hot sauce.
