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Stir together lemon juice, cumin seeds, salt, and pepper in a medium bowl.
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Stir in oil and let stand while roasting beets.
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Put oven rack in middle position and preheat oven to 425°F.
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Tightly wrap beets in a double layer of foil and roast on a baking sheet until tender, 1 to 1 1/4 hours.
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Cool to warm in foil package, about 20 minutes.
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When beets are cool enough to handle, peel them, discarding stems and root ends, then cut into 1/2-inch-wide wedges.
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Toss warm beets with dressing.
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Stir in mint just before serving.