Curry Scented Butternut Squash Soup

1 butternut squash, 2 1/2 lbs (6 cups cubed)
2 large sweet apples (fuji or honeycrisp)
3 medium onions
2 tbsp butter
1/4 cup packed dark brown sugar
1 1/2 tbsp curry powder
6 cups chicken or vegetable stock
1 8 oz container mascarpone cheese
2 tsp salt
sour cream and snipped fresh chives (optional)

*Peel squash and apples. cut into 1 inch cubes and dice onions.
*Melt butter in large 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. Simmer 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 a large bowl. Repeat two times with remaining squash mixture.
*Return soup to pot and whisk in mascarpone and salt, until mascarpone is completely dissolved. Return to simmer over medium heat and remove from heat. garnish each serving with sour cream and chives if desired.