I've never made flank steak before, but we used to use a variation of this marinade on chuck steak "back in the day." I thought it might work, and I was right! What a delicious dinner we had. Thanks for the contest this week. I conquered a new challenge. ;) Mushrooms just seemed a natural continuance of the umami factor in the marinade, and it made a great sauce.
Because I live on the 5th floor, I have no grill, so I'm a big fan of the grill pan. Feel free to do this on your grill outdoors, however. —ChefJune
Combine the marinade ingredients -- that's everything down to the steak -- in a jumbo zip lock freezer bag. Wipe the meat and place it in the bag. Coat the steak well with the marinade. Seal the bag and place in a bowl. Chill and marinate for at least 6 hours. (Overnight is better.)
Slice the mushrooms and prep a small pan to saute them.
Heat grill pan over medium high heat until it is so hot you can hardly hold your hand over it.
Take the steak out of the marinade bag and dry it very well. Be sure to save the marinade as it will become sauce with the mushrooms. Place steak on the hot grill pan. Grill for 4-6 minutes on each side. Do not move the steak while it is cooking!
You want your steak done to medium rare; well done will make it tough. When the second side has cooked 4 minutes, remove from the grill and place on a warm platter. Cover with aluminum foil to hold in the heat and to keep the steak from drying out, and let rest for 10 minutes.
While the steak is resting, add the marinade to the now sauteed mushrooms and bring to a boil. Reduce heat and allow the liquid to reduce to 1/3 its previous amount
Remove the steak to a cutting board and slice thinly against the grain, and at a slight diagonal so that the slices are wide. Stir the juices from the platter into the mushrooms and sauce. Return the sliced steak to the platter and nap with several spoonsful of mushrooms and sauce. Serve the remaining sauce on the side.