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Mix garlic, lemon zest, oregano, rosemary, 1/2 teaspoon of salt, and 1/2 cup of oil into lemon zest. Season marinade to taste with black pepper.
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Toss shrimp in a medium bowl with 3 tablespoons of marinade to coat. Cover and refrigerate for at least 1 hour and up to 1 day. Cover and refrigerate remaining marinade; return to room temperature before grilling.
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Prepare barbecue for medium-high heat.
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Thread shrimp onto flat metal skewers, spacing shrimp apart so they cook evenly.
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Gently toss zucchini, squash, asparagus, and onions in a large bowl with 3 tablespoons of remaining marinade to coat. Thread tomatoes onto additional flat metal skewers.
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Brush grill rack with oil. Grill zucchini, squash, and onions until golden brown grill marks form and the vegetables are crisp-tender, about 10 minutes. Grill tomatoes until they begin to bursting open and caramelize, about 10 minutes.
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Grill asparagus until grill marks form and asparagus is crisp-tender, about 4 minutes. While vegetables grill, brush some of remaining marinade over them and turn vegetables as needed.
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Transfer vegetables to platter and tent with foil to keep warm.
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Grill shrimp, brushing with remaining marinade, until they are just cooked through and pink, about 2 minutes per side for extra-large shrimp or 3 minutes per side for colossal shrimp.
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Squeeze juice from 1 lemon wedge over shrimp on grill.
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Transfer shrimp to platter with vegetables and serve with remaining lemon wedges.