Cooking the sweet potatoes on a grill retains vitamins that would be lost if they were boiled, and gives them a crisp, charred edge that contrasts with the sweet interior. Tossing them with the lemon-olive oil dressing brightens the flavor.
Preheat grill to medium heat.
Peel sweet potatoes, and cut diagonally into 1/2-inch-thick slices. Place sweet potato slices on grill rack coated with cooking spray; grill 18 minutes or until tender, turning once. Place onions on grill rack during last 2 minutes of cook time; cook onions 2 minutes or until browned and tender, turning once. Cut onions into 2-inch pieces.
Combine olive oil and next 4 ingredients (through red pepper) in a large bowl, stirring with a whisk. Add sweet potato and onions; toss well.
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