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 up beef; 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 cheese.
Cook dip, covered, on low 1-3/4-2-1/4 hours or until mixture is heated through and cheese is melted. Top with tomatoes and pickles. Serve with tortilla chips or crackers.
Test Kitchen tips
Keep an eye on the dip toward the end of cooking. If it goes too long, the edges will get dark.