Heat grill. In small skillet, cook onions over medium-high heat for 3 to 4 minutes or until slightly browned, stirring occasionally. Set aside.
When ready to grill, cut franks in half lengthwise, cutting almost but not completely through. Open franks and place, cut side down, on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 5 to 10 minutes or until thoroughly heated, turning once. To toast buns, place cut sides down on grill during last 1 to 2 minutes of cooking time.
To serve, spread cut sides of each bun with about 1/2 tablespoon relish. Place frank in each bun. Top with salad dressing and onions.