Place bacon in a large, deep skillet. Cook over medium high heat until evenly brown. Drain, crumble and set aside.
Place the bacon, Cheddar cheese, onion, mayonnaise, and mustard into the bowl of a food processor; process until well blended. Scoop into a container, cover, and refrigerate several hours to overnight to allow the flavors to blend.
Adjust oven rack to the topmost position, and set oven to Broil.
Spread the bread rounds evenly with cheese mixture and place onto a cookie sheet. Toast under the broiler until browned and puffed, 5 to 8 minutes. Serve immediately.