Scrub the potatoes. In a medium saucepan over medium heat, boil potatoes in salted water until barely tender, about six to eight minutes. Drain and cool. When cool enough to handle, slice potatoes in half.
In a medium mixing bowl, whisk together mustard, oil, onion, rosemary, garlic, sugar and paprika. Add potatoes and toss to coat.
Preheat grill for medium-high direct cooking. Cook potatoes for eight to 10 minutes or until browned, turning often.