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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Trim, peel, and cut carrots on a diagonal into ½-inch-thick pieces. Halve, peel, and cut onion into ½-inch-thick wedges. Strip rosemary leaves from stems; finely chop leaves until you have 2 tsp (4 tsp for 4). Halve, core, and cut apple into ½-inch-thick wedges.