Preparation / Directions:
Prepare grill or heat oven to 450ø. Make a tray from heavy-duty aluminum foil by folding foil over for double strength and folding up sides; oil tray. Cut potatoes into pieces no larger than 1 1/2 inches.
Combine the potatoes, garlic, 1 tbsp olive oil and 1/4 tsp salt in medium-size bowl; toss to coat potatoes. Spoon potato mixture into foil tray, keeping potatoes in one layer. Top with rosemary sprigs.
To grill: Grill the potato mixture 5 inches above medium-hot coals, covered, turning occasionally, for 25-30 minutes or until potatoes are tender and crisply browned.
To roast: Roast potato mixture in 13 x 9 x 2-inch baking pan in 450ø oven, stirring occasionally, 30-40 minutes or until tender and browned.
Meanwhile, whisk vinegar, mustard, remaining 1/4 tsp salt and the pepper in medium-size bowl. Whisk in remaining 3 tbsp oil. Discard woody rosemary stems from grilling tray. Squeeze garlic cloves from skins into dressing in bowl. Mash slightly into dressing.
Add potatoes and onions to dressing; toss to coat.