How to Make It
Preheat grill to high.
Coat cut sides of squash and onion with cooking spray; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Arrange squash and onion, cut side down, on grill; cover and grill 5 minutes. Rotate vegetables; cover and grill 3 minutes. Turn vegetables over; grill 2 minutes. Remove squash from grill. Continue grilling onion 4 minutes; remove from grill. Cool to room temperature.
Coarsely chop vegetables; place in a large bowl. Add remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, oil, and juice; toss to coat. Add basil, mint, and parsley; toss.