Chop mushrooms fine (save some for garnish) and simmer with onion for 30 minutes in broth. Melt 6 T of butter in a small saucepan (different from other mixture). Add flour and stir with whisk until blended. Bring milk to a boil and add it to the roux (butter/flour). Add cream, stir until thickened. Add broth and with vegetables. Just before you serve, season and add sherry to taste.