In a large skillet, cook beef over medium heat until no longer pink, 6-8 minutes; crumble meat; drain. Stir in ketchup, mustard, bacon and Worcestershire sauce; bring to a boil. Reduce heat; simmer, uncovered, until slightly thickened, 5-7 minutes, stirring occasionally.
Meanwhile, in a small bowl, combine mayonnaise, milk, relish and pepper; set aside.
Preheat griddle over medium heat. Sprinkle 4 tortillas with cheese; top with beef mixture and remaining tortillas. Place on griddle; cook until golden brown and cheese is melted, 1-2 minutes on each side. Serve with sauce and, if desired, lettuce and tomatoes.
1 quesadilla with about 1/4 cup sauce: 1002 calories, 60g fat (17g saturated fat), 110mg cholesterol, 2115mg sodium, 75g carbohydrate (18g sugars, 4g fiber), 39g protein.