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Adjust rack to middle position (middle and top positions for 4 servings) and preheat oven to 425 degrees. Bring a medium pot of salted water to a boil. Wash and dry produce.
Trim, peel, and halve carrots lengthwise; cut crosswise into 2-inch-long pieces. Trim and thinly slice scallions, separating whites from greens. Peel and mince or grate garlic. Zest and quarter lemon.
Trim and halve Brussels sprouts lengthwise or trim and discard woody bottom ends from asparagus. (Save carrots for another use.)