Smooth and silky butternut squash soup is a must-have for cold weather. The secret to making the best butternut squash soup you have ever had is to roast the squash until deeply browned and caramelized in the oven, then purée it with a base of browned butter and sautéed onions. A dollop of sour cream, cinnamon, nutmeg-spiced whipped cream, and frizzled sage makes this a wintertime classic.