Combine miso and 1 tablespoon water. Gradually add oil, stirring with a whisk. Place zucchini, eggplant, and bell peppers on a jelly-roll pan. Add 5 tablespoons miso mixture; toss to coat. Brush onion with remaining miso mixture.
Place vegetables on a grill rack coated with cooking spray. Grill zucchini, eggplant, and bell pepper 4 minutes on each side or until tender.
Grill onion 6 minutes on each side or until tender. Sprinkle with mint. Serve with lime wedges.