Divide bread cubes evenly among 10 (8- to 10-oz.) ovenproof coffee mugs coated with cooking spray, placing in bottom of mugs. Top evenly with sausage. Whisk together 2 1/2 cups milk, eggs, and Dijon mustard. Pour evenly over bread mixture in mugs.
Whisk together buttermilk and cream of mushroom soup. Spoon over bread mixture in mugs; sprinkle with Cheddar cheese. Place coffee mugs on a baking sheet.
Bake at 350° for 25 to 30 minutes or until casseroles are set and puffed. Serve immediately.
Note: Unbaked mugs of casserole can be covered with plastic wrap, then foil, and frozen up to 1 month. Thaw overnight in the refrigerator. Bake as directed.
Sausage-and-Egg Casserole: Omit coffee mugs. Arrange bread in 2 lightly greased 8-inch square baking dishes or 1 lightly greased 13- x 9-inch baking dish. Proceed as directed, increasing bake time to 1 hour or until casserole is set. 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.