Place the potatoes into a large microwavable bowl and microwave on high until just starting to soften, about 10 minutes, stopping to stir every once in a while. Remove the bowl carefully as it will be hot.
Combine 2 tablespoons of the olive oil, the garlic, salt and pepper and the rosemary. Pour over the potatoes and toss to coat.
Transfer potatoes to a grill basket (I put the basket over a plate or bowl to catch any dripping oil mixture), or thread them onto skewers.
Place onto the grill and grill until lightly charred, crispy on the outside and tender on the inside, turning occasionally
Meanwhile, pour the balsamic vinegar into a small saucepan over high heat and reduce until you are left with two tablespoons, stirring often.
Pour remaining olive oil onto a serving platter and spread out evenly. Sprinkle with salt and pepper.
Add the potatoes.
Drizzle with the reduce balsamic vinegar and serve.