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Prepare grill for direct-heat cooking, medium-high heat. Whisk together garlic, soy sauce, vinegar, oil, salt, pepper and herbs in a large bowl; set aside 2/3 of marinade for brushing.
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Halve onion lengthwise and cut into 1/2-inch cubes. Thread vegetables onto skewers (with small spaces between for even cooking).
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Bring a large pot of water to a boil and add salt. Set up a large bowl with ice and water to create an ice water bath.
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Blanch skewers with vegetables for 1 minute in boiling, salted water. Remove skewers and put in ice water. When vegetables have cooled completely, drain well on paper towels and toss in reserved marinade.
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Oil rack and carefully place skewers on the grill. Cover only if using a gas grill, turning occasionally with tongs and continuously brushing with marinade, until vegetables are very tender, 6 to 10 minutes. Serve warm with fresh tomato sauce.