Preheat oven to 425°F. Fill a bowl with ice cubes and water. Cook carrots in boiling salted water for 6 minutes. Add cauliflower; cook 5 minutes. Drain; pour into ice bath. Drain again.
Mist a 9-by-13-inch dish with cooking spray. Mix onion, garlic, horseradish, mayo, sour cream, salt and pepper. Stir in vegetables. Put into baking dish.
Stir butter into crumbs; sprinkle over vegetables. Cover; bake for 20 minutes. Uncover; bake for 5 minutes. Sprinkle with parsley; serve.