In a large skillet set over medium heat, melt 4 tbsp butter. Add shallots and garlic and saute until softened and just starting to brown.
Add mushrooms and cook, stirring often, until mushrooms release their juices. Continue cooking until rendered juices have evaporated then add brandy and white wine. If cooking over a gas fire, be careful of the alcohol catching fire. Cook until almost all of the liquid has evaporated then add mushroom stock, thyme leaves and tarragon and reduce again.
Once mushrooms are well cooked and liquid is almost entirely reduced. Remove from heat then transfer to a food processor and let cool slightly.
Add remaining 2 tbsp butter, cream cheese and red wine vinegar and puree until smooth. Season with salt and pepper to taste and transfer to a serving dish.
Serve spread on crackers or sliced baguette with cornichons, pickled onions and grainy mustard.
