In a large pot, sauté onion in butter until softened. Add chicken broth and bring to a simmer. Simmer for 30 minutes. Strain onion from broth and discard
Whisk in puree, brown sugar and spices. Bring to a simmer and simmer 10 minutes, stirring occasionally to prevent sticking
Add cream and heat through, about 5 minutes, making sure not to let soup boil.
Notes
Make it vegetarian by substituting vegetable broth for the chicken broth.
Soup can be frozen. Prepare without adding cream. Cool and place in freezer safe container. Defrost in refrigerator for 12-24 hours. Heat soup until hot and add cream as directed in final step.