Prepare a grill for medium-high heat; lightly oil grate. Whisk garlic, vinegar, and ½ cup oil in small bowl to combine; set marinade aside.
Toss squash, onion, and bay leaves on a rimmed baking sheet with remaining 3 Tbsp. oil to coat; season generously with salt and black pepper.
Arrange squash and onion on grate. Grill squash, undisturbed, until grill marks appear, about 3 minutes. Turn over and grill on second side until tender and starting to release liquid, about 2 minutes. Transfer squash back to baking sheet. Grill onion, turning occasionally, until tender and charred around the edges, about 5 minutes. Transfer back to baking sheet.
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