Grilled Potatoes

Bahan-bahan (6 porsi)

• 3 pounds red potatoes
• 1 small onion (sliced)
• ¼ cup butter (cubed, more to taste)
• 3 -4 cloves garlic (sliced)
• 2 teaspoons fresh rosemary (chopped, or ½ teaspoon dried and slightly crushed)
• salt and black pepper (to taste)