Brown meat with onions in large nonstick skillet on medium-high heat; drain. Return meat mixture to skillet. Add 1/2 cup ketchup and 2 Tbsp. mustard; mix well. Cook 2 min., stirring frequently. Remove from heat.
Tear 2 Singles into small pieces. Add to meat mixture; mix well. Cut each of the remaining Singles into 4 pieces each.
Cut rolls horizontally in half. Press 1 roll half, cut side up, onto bottom and up side of each of 24 mini muffin pan cups sprayed with cooking spray.
Spoon meat mixture evenly into bread shells; top each with Singles piece.
Bake 10 to 12 min. or until Singles are melted.
Drizzle with remaining ketchup and mustard; top with relish.