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Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Halve, peel, and finely chop onion. Trim and quarter mushrooms (skip if your mushrooms are pre-sliced!). Trim, peel, and halve carrots lengthwise; slice crosswise into ¼-inch-thick half-moons. Finely dice celery. Peel and mince or grate garlic.