Mashed Potatoes with horseradish

3 lbs russet potatoes or 3 lbs yukon gold potatoes , peeled and quartered 6 garlic cloves 1 small onion , chopped crushed red pepper flakes 1/4 cup butter ,[...]

4 russet potatoes , about 2 lb total, peeled and cut into rounds 1/4 inch thick 3 parsnips , about 3/4 lb total, peeled and cut into rounds 1/4 inch thick 4[...]