Arrange bread in 2 lightly greased 8-inch square baking dishes or 1 lightly greased 13- x 9-inch baking dish. Top evenly with sausage. Whisk together 2 1/2 cups milk, eggs, and Dijon mustard. Pour evenly over bread mixture.
Whisk together buttermilk and cream of mushroom soup. Spoon over bread mixture; sprinkle with Cheddar cheese. Place casserole on a baking sheet.
Bake at 350° for 1 hour or until casserole is set. Serve immediately.
Note: An unbaked casserole can be covered with plastic wrap, then foil, and frozen up to 1 month. Thaw overnight in the refrigerator. Bake as directed.
Use 1 (16-oz.) package of crumbled pork sausage instead of the patties, if desired. Simply cook in a nonstick skillet until browned and crumbled.