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 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 and green onions (dish will be full).
In a bowl, whisk together eggs, whipping cream, dry mustard, salt, and pepper. Gradually pour egg mixture over layers in dish. Cover with plastic wrap and chill 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.