Preheat oven to 375 degrees F.

Place squash in oven for about 15 minutes to soften somewhat, then peel and slice into bite-sized pieces, discarding seeds.

Combine squash in a roasting pan with potatoes and apple. In a small bowl, whisk together horseradish, olive oil and salt and pepper. Pour mixture over vegetables and apple and toss. Roast uncovered, tossing occasionally, until deeply roasted and caramelized, about 1 hour. Serve.