Preheat oven to 375°F. In a large skillet over medium-high heat, cook sausage until cooked through and no longer pink, breaking it up into crumbles as it cooks.
Spray a 9 x 13 inch baking dish with nonstick cooking spray or grease with butter. Unroll crescent roll dough into pan and press together seams and perforations. Sprinkle cooked sausage on top of dough, followed by both types of cheese.
In a large mixing bowl, whisk together eggs, milk, ground mustard, and black pepper. Pour over sausage and eggs.
Bake for 35 to 40 minutes or until set and no longer liquid in the middle.
Let stand for 5 to 10 minutes before slicing and serving.
Notes
Store leftovers in a covered container in the fridge for 2 to 3 days.
Add vegetables: Add chopped broccoli (one head) to the casserole. There's no need to cook the broccoli first. Use frozen or fresh. Other great add-ins are red or green bell peppers, sweet onions, spinach, or asparagus. Mushrooms are delicious, too, but brown those with sausage before adding them to the casserole.
Instead of turkey sausage: Substitute pork sausage, chorizo, cubed ham, or crumbled cooked bacon.
Make it vegetarian: Omit the meat and add your choice of veggies instead.