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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Cut potatoes into ½-inch-thick wedges. Halve, peel, and thinly slice ¼ of the onion; finely dice remaining onion. Halve lime. Halve buns.
Toss potatoes on a baking sheet with a large drizzle of oil, salt, and pepper.
Roast on top rack until browned and tender, 20-25 minutes.