Scrape carrots and cut on diagonal, making slices about ¼-inch thick. Cook in salted water until tender but still crisp. Drain, reserving ¼ cup cooking liquid. Mix onion, horseradish, mayonnaise, salt, pepper, and carrot liquid; combine with carrots in a buttered casserole. Mix the bread crumbs with the melted butter and spread over the carrots. Bake at 375 degrees F for 15 to 20 minutes or until bubbly.