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Put oven rack in upper third of oven and preheat oven to 475°F.
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Cut parsnips lengthwise into quarters, then cut out and discard any tough core. Cut parsnips diagonally into 2-inch pieces. Cut squash crosswise into 1/2-inch-thick slices, then cut slices into wedges. Toss vegetables with salt, pepper, and 1 1/2 tablespoons oil, then spread in 1 layer in a large shallow baking pan. Roast, turning occasionally, until vegetables are tender and browned, 20 to 25 minutes.
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While vegetables are roasting, heat remaining 1 1/2 tablespoons oil with garlic in a 1-quart saucepan over low heat, stirring occasionally, until garlic just begins to sizzle. Remove from heat.
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When vegetables are done roasting, remove garlic from oil (discard garlic) and toss vegetables with garlic oil, olives, and parsley.