* Heat 2 tablespoons of butter in a large pot until bubbling. Add leeks and cook until wilted but not browned, about 5 minutes.
* Add mushrooms and cook for one minute longer.
* Stir in sherry and let simmer until reduced by half. Then add chicken stock and bring to boil.
* Mix the remaining 6 tablespoons butter with 6 tablespoons of flour to from paste. Wish into the boiling soup until well combined. Bring to boil, lower to simmer for 10 minutes, skimming away any foam that floats to the surface.
* Turn the heat to low and stir the Brie in a few chunks at a time. Adjust seasoning with salt and pepper. Serve garnished with diced tomatoes and chives.