Place the potatoes in a large saucepan. Cover with cold water to 2 inches above potatoes; bring to a boil. Cover, reduce heat, and simmer 8 minutes or until almost tender. Drain and rinse with cold water; drain. Combine potatoes, 2 teaspoons oil, 1/8 teaspoon salt, 1/8 teaspoon pepper, and onion in a large bowl, tossing gently to coat. Arrange potatoes and onion in a single layer on grill rack coated with cooking spray; grill 10 minutes or until potatoes are tender, turning once.
Combine the remaining 1 tablespoon oil, remaining 1/2 teaspoon salt, remaining 1/4 teaspoon pepper, sherry vinegar, chicken broth, and minced garlic in a large bowl, stirring with a whisk. Add potato mixture to vinegar mixture; toss gently to coat. Let stand 20 minutes. Stir in parsley.