In a large skillet, cook bacon over medium heat until crisp. Drain bacon on paper towels; discard drippings. (If using sausage, cook over medium-high heat until brown. Drain off fat.)
Meanwhile, grease a 2-quart oval or square baking dish; set aside. Spread one side of bread slices with butter. Cut bread into 1-inch cubes. Place half of the bread cubes in the prepared baking dish. Sprinkle with half of the bacon. Top with the remaining bread cubes and the remaining bacon. Sprinkle with cheese (dish will be full).
In a medium bowl, whisk together eggs, whipping cream, dry mustard, salt, and pepper. Gradually pour egg mixture over layers in dish. Chill, covered, in the refrigerator for 2 to 24 hours.
Preheat oven to 325 degrees F. Bake, uncovered, for 50 to 55 minutes or until center is set (170 degrees F). Let stand for 10 minutes before serving. If desired, sprinkle with parsley. Makes 6 to 8 servings.