1. Preheat the oven to 200°. Coat a jelly-roll pan with cooking spray.
2. Rinse grapes; drain well, leaving slightly moist. Combine grapes and sugar in a large bowl; toss to coat. Arrange grapes on prepared jelly-roll pan. Bake at 200° for 3 hours or until grapes soften but still hold their shape. Remove from oven; cool completely.
3. Spoon 1/4 cup yogurt into the bottom of each of 4 parfait glasses. Top each serving with about 1/3 cup grapes, 1 1/2 teaspoons walnuts, and 1 teaspoon honey. Repeat layers with remaining ingredients; garnish with sliced mint, if desired.