2 1/2 lbs. medium sweet potatoes, peeled and cut in half
1/4 C. (1/2 stick) butter
1/4 of a medium Spanish onion, diced
1 quart water
1 pint whipping cream
1 pint whole milk
1 T. chicken base
1/2 lb. dark brown sugar (about 1 1/4 C. packed; see note)
1 t. ground cinnamon
1 pinch ground nutmeg
1 t. salt
1/2 t. white pepper
4 T. butter
4 T. flour
Boil sweet potatoes until tender. Drain, cool slightly, then puree in batches in food processor. Set aside.
Melt butter in large soup pot over medium heat. Saute onions until tender. Add sweet potatoes and cook just until heated through. Blend in water, cream, milk, chicken base and sugar. Bring to boil. Add seasonings.
Make roux by melting butter in small saucepan. Gradually whisk in flour and cook 2 to 3 minutes, whisking occasionally. Gradually whisk some of the roux into soup, stirring until smooth. Simmer until thick, 2 to 3 minutes. Soup should be thick enough to coat back of spoon. Make and add more roux if needed. Taste soup and adjust seasonings.