Marinade: mix the dark beer, 1/2 liter burgundy wine, bay leaves and crushed garlic - set aside.
Make two slits in each filet midway in the cut, about 3" long and not quite cut all the way on top or bottom. Stuff these with slices of mushroom, and bacon pieces; salt and pepper to taste.
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 ½ cup butter in a small saucepan. Stir in flour and brown. Blend in the beef stock and ½ cup of marinade until smooth. Stir in mushrooms and heat through. Remove from heat and stir in ½ cup burgundy. Pour sauce over filets.