Preheat grill to medium-high heat. Layout two sheets of foil about 12-14 inches long each. (Double them up). Spray the top sheet with non-stick cooking spray. Spread the potatoes and onion evenly on to the foil. Sprinkle with cheese, garlic salt, rosemary, and pepper. Dot with butter. Fold over sides and end of the foil and seal tightly. Place on the grill for 40-45 minutes or until the potatoes are tender. Remove from the grill and place on a baking sheet. Cut open foil and serve immediately.