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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Dice potatoes into 1-inch pieces. Halve, peel, and mince half the onion; cut remaining onion into quarters. Trim, peel, and cut carrots on a diagonal into ½-inch-thick pieces.
Toss potatoes, quartered onion, and carrots on a baking sheet with a large drizzle of oil, salt, and pepper. Roast on top rack until browned and tender, 20-25 minutes.