Potato-Cauliflower Salad with Marinated Red Onions

Whisk olive oil, horseradish and white wine vinegar in medium bowl to blend. Mix in onion. Season generously with salt and pepper. Let stand for 2 hours at room temperature or cover and refrigerate up to 1 day, tossing occasionally. Steam cauliflower until just tender, about 4 minutes. Cool 5 minutes, then add to red onion mixture. Steam potatoes until just tender, about 20 minutes. Cool 15 minutes. Slice potatoes thickly. Place in large bowl. Add onion mixture to potatoes. Add blue cheese; toss lightly to blend. Season to taste with salt and pepper. (Can be prepared 2 hours ahead. Cover and let stand at room temperature.)