1. Melt butter in pot over medium low heat. Add onions and garlic and sautee until onions are softened.
2. Add mushrooms and cook until most mushroom liquid is evaporated. Add white wine and reduce.
3. Add flour and mix well, continue cooking for another minute. Add stock slowly while stirring. Add tarragon sprigs and bring to a slow boil while continuously stirring.
4. Reduce heat to medium low and cook for an additional 30 minutes while periodically stirring. Remove from heat, add cream and adjust seasonings with salt and pepper. Remove tarragon sprigs.