Trim the rind from the brie and then roughly tear it into 2-inch pieces. (It is easy to trim if you first place the cheese in the freezer for 30 minutes or until it is sufficiently hard.) Set aside.
In a small saucepan, reduce the sherry over medium-high heat to 1 cup. Set aside.
in a medium stockpot over medium-high heat, melt the butter. Add the mushrooms, shallots, and lemon juice and cook 4 minutes, stirring constantly. Remove from the heat, add the flour, and stir until the flour is fully incorporated.
Return the mixture to heat and add the beef broth and reserved sherry. Raise the heat to high and bring the mixture to a boil. Reduce the heat and simmer for 15 minutes.
Add the trimmed Brie, stirring until the cheese has melted. Add the half and half and pepper and continue to simmer without boiling for 5 minutes.
ladle the soup into separate bowls and garnish each with 1 to 2 tablespoons of the chives.