Using Serrated Peeler, peel squash and apples; cut into 1-in. (2.5-cm) cubes and dice onions.Melt butter in All-Purpose Pot over medium-low heat. Add onions; cook 5-6 minutes or until tender, stirring occasionally. Add squash, apples, brown sugar and curry powder. Cook and stir 1 minute or until curry powder is fragrant. Add stock and bring to a simmer over medium heat. Cook 24-26 minutes or until squash is very soft. Remove from heat.Carefully ladle about one-third of the squash mixture into blender container. Cover and blend until smooth. Pour blended soup into Stainless (6-qt./6-L) Mixing Bowl. Repeat two times with remaining squash mixture.Return soup to Stockpot and whisk in mascarpone and salt until mascarpone is completely incorporated. Bring to a simmer over medium heat and remove from heat. Garnish each serving with sour cream and chives, if desired.