Grilled New Potatoes

1 small onion, sliced
1 1/2 pounds new white potatoes, scrubbed
1/4 C melted butter or margarine
1/2 tsp salt
1/4 tsp pepper
1/4 tsp garlic salt
1/2 tsp dried rosemary

Preheat grill or broiler. Spray a large piece of heavy-duty foil generously with cooking spray. Arrange onion slices in center of foil. Cut potatoes into halves; arrange cut sides up on onion slices. Drizzle butter over potatoes. Sprinkle with salt, pepper, garlic salt and rosemary. Fold foil to enclose vegetables; seal tightly. Grill or broil 40 minutes, turning once. Open foil and fold back edges. Grill 5 minutes longer or until potatoes are tender.