MIX meat and mustard until blended; shape into 16 (1-1/2-inch) patties.
BROIL, 6 inches from heat, 6 to 8 min. or until done (160ºF), turning after 4 min. Top with Singles pieces; broil 1 min. or until melted.
PLACE cheeseburgers on 16 crackers; top with avocados, ketchup, mayonnaise and remaining crackers to make sandwiches.