Make a tray from doubled aluminum foil; fold up sides.
Oil the tray.
Cut potatoes into pieces no larger than 1 1/2 inches.
Combine potatoes, garlic, 1 T. oil and 1/4 teaspoons salt in medium bowl.
Toss til potatoes are well coated.
Spoon potatoes into the oiled tray.
Keep potatoes in a single layer.
Top with rosemary sprigs.
To grill: Grill potatoes 5 inches above medium-hot coals, covered with dome, turning occasionally, for 25-30 minutes or til tender and crisply browned.
To roast: Roast potatoes in preheated oven, stirring occasionally, 30-40 minutes or til tender and browned.
Meanwhile, whisk together vinegar, mustard, remaining salt and pepper in a medium bowl.
Gradually whisk in remaining oil.
When potatoes are done, discard rosemary.
Squeeze grilled garlic cloves from skins into bowl.
Slightly mash garlic into dressing.
Add potatoes and green onions to dressing; toss gently to coat.