Start a fire in your grill outside.  Melt the butter in a skillet over medium heat.  Add the shallots, mushrooms, and tomatoes and sauté for about 5 minutes.  Salt and pepper to taste.  Increase to high heat and add the wine, beef  stock and tarragon (this is when it is nice to have some "Homemade Stock" in the freezer, but canned stock will do in a pinch).  Bring back to a boil and reduce to low heat and simmer while you prepare the meat, adding additional stock when the liquid boils down too low.  Season the flank steak with season salt and pepper and transfer to the grill.  When the meat is done, take it off the fire and finish the sauce.  Blend the arrowroot and water in a small bowl.  Stir the arrowroot mixture into the sauce until the sauce thickens and remove the skillet from the heat.  Slice the meat and serve topped with generous portions of Sauce Chasseur.