1. Stir the olive oil, garlic, and rosemary together in a small bowl and let it sit at room temperature for at least 15 minutes for the flavors to infuse.
2. Preheat a grill to medium-high.
3. Place the potatoes in a pot of salted water and bring to a boil. Reduce the heat to a simmer and cook until the potatoes are tender when pierced with the tip of a knife, about 12 minutes. Drain the potatoes, discarding the water.
4. Using a towel wrapped around the palm of your hand, gently smash each potato until the skin breaks, while trying to keep the potato whole (leaving it about 1 inch thick). Transfer the potatoes to a baking sheet. Generously brush both sides of the potatoes with the olive oil mixture, and season both sides with the salt and pepper.
5. Place the potatoes on the grill and cook until grill marks appear and the potatoes are nicely caramelized, 3 to 4 minutes per side.
Yield: 4 to 6 servings