2 pounds assorted summer squash (such as zucchini and summer squash), halved lengthwise
1 large fresh red onion, halved
1/2 teaspoon kosher salt, divided
1/2 teaspoon freshly ground black pepper, divided
1/4 cup extra-virgin olive oil
2 tablespoons fresh lemon juice
1/2 cup torn fresh basil leaves
1/2 cup torn fresh mint leaves
1/2 cup fresh flat-leaf parsley leaves
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.