Gingered Butternut Squash Soup

1 1/2 lb. carrots, peeled and cut in large chunks
1 lb. butternut squash, peeled, seeded and cut into chunks
1 large onion, quartered and sliced
1 3-in. piece of fresh ginger, peeled and chopped
2 T. butter
3 T. dark brown sugar
1/4 to 1/2 t. kosher salt
4 cups low-sodium chicken broth
large pinch cayenne pepper
Nonfat plain Greek yogurt for serving

In a slow cooker combine all of the ingredients except the yogurt and cook on LOW for 4 hours. Process until smooth in either a food processor or with an immersion blender. Adjust seasonings and serve with a dollop of Greek yogurt.