Sirloin and Horseradish Cream Sauce with Garlic Bread and Roasted Carrots

Adjust rack to middle position and preheat oven to 425 degrees. Wash and dry all produce. Trim and thinly slice scallions, separating whites from greens. Trim and peel carrots, then slice into ½-inch-thick pieces on a diagonal. Halve, peel, and thinly slice onion.