In a large skillet, cook beef and onion over medium-high heat until beef is no longer pink and onion is tender, 6-8 minutes, breaking beef into crumbles; drain. Transfer to a greased 1-1/2- or 3-qt. slow cooker. Stir in the cream cheese, 1-1/2 cups cheddar cheese, Worcestershire, mustard, salt and pepper. Sprinkle with remaining 1/2 cup cheese., Cook dip, covered, on low until mixture is heated through and cheese is melted, 1-3/4 to 2-1/4 hours. Top with chopped tomato and pickles. Serve with tortilla chips or crackers.