In a large saucepan, cook onion, curry powder, cumin, coriander, and salt in hot butter until onion is tender. Whisk in pumpkin, chicken broth, and brown sugar or maple syrup, if desired, until well combined. Bring mixture just to boiling; reduce heat. Stir in half and half and heat through. If desired, garnish with sour cream and sage leaves. Makes 6 to 8 servings.