Heat oven to 375ºF. Spread vegetables in large baking dish, drizzle with olive oil and cider or wine, scatter on the garlic, sprinkle with salt and pepper to taste, and toss well. Cover the dish tightly with foil and bake 20 minutes, then remove foil and continue to roast until vegetables are brown-tipped and tender, 20-35 minutes longer. Meanwhile, make a white sauce by melting the butter in a saucepan; add the flour and cook over low heat, stirring, for 3-5 minutes. Whisk in the warm milk, bring sauce to a simmer and cook gently about 10 minutes, stirring often. Season well with salt and pepper; stir in the nutmeg, then fold in the vegetables. Transfer to a buttered casserole dish or leave in the roasting pan. Mix horseradish, Cheddar and breadcrumbs with your fingers and sprinkle evenly over the vegetables. Continue to bake until bubbly, 20-30 minutes longer.