Spray 9×13 casserole dish with nonstick cooking spray and set aside.
Crumble sausage in a medium skillet with onion and garlic. Cook over medium heat until the sausage is browned and cooked through and the onion is tender. Drain and set aside.
Arrange bread cubes in bottom of prepared casserole dish.
Stir together sausage mixture, milk, eggs, cheese, salt, pepper, and dry mustard and pour over cubed biscuits. Cover tightly and refrigerate overnight up to two days.
Preheat oven to 350° F and bake for 40-45 minutes, until center of casserole is just set. Allow to stand for about 10 minutes before serving.