1
Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Cut potatoes into 1⁄2-inch-thick wedges. Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Halve, peel, and thinly slice onion.
In a small bowl, combine mayonnaise with horseradish sauce; season with salt and pepper.