Sauté onions in 4 T butter until soft. Remove from pan.melt remaining butter; whisk in flour; cook 3 min constantly whisking without browning. Stir in onions and gradually add broth and 1/2 and 1/2, whisking constantly. Bring to boil and simmer x 5 min. Stir in everything except sour cream. Stir in sour cream 20 min before serving.
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