Soak each filet in the prepared Guinness-Burgundy-Brandy Marinade. Refrigerate for 24 hours or more. Remove and drain filets when ready to grill. Grill filets mignons for 3 to 5 minutes per side, which will cook them to medium or medium-rare doneness. Add salt and pepper to taste. Near the end of cooking, top with a slice of Brie so it melts for a nice presentation.
To make a sauce to cover meat, melt the ˝ cup butter in a small saucepan. Stir in flour and brown. Blend in the beef stock and ˝ cup of marinade until smooth. Stir in mushrooms and heat through. Remove from heat and stir in ˝ cup burgundy. Pour sauce over filets.