Combine red wine marinade ingredients in a small bowl. Place flank steak into a resealable plastic bag. Pour marinade over top making sure all of the steak is well submerged in the liquid. Seal bag and place into the refrigerator for 4-12 hours.
Clean mushrooms and cut each one in half, lengthwise. Slice onion into long strips.
Preheat grill for medium high heat. Place cast iron skillet onto grill to heat up for 10 minutes before adding the oil.
Remove steak from bag and place onto grill. Cook for 8-10 minutes per side based on thickness of cut and your desired doneness. Recommended for this dish: an internal temperature of 140-145 degrees F for medium rare, or 150-155 degrees for medium. Move steak off to the side of the pan, do not remove from the pan.
Heat oil in pan and add onions. Cook for 3-4 minutes or until they start to become tender. Add mushrooms, season with salt and black pepper and cook for an additional 3-5 minutes.
Remove both the steak and vegetables from grill. Set cast iron pan onto a heat safe surface. Carve steak into thin slices and serve with onions, mushrooms, and your favorite side dish.
This recipe also works really well on a flat top grill or on your stovetop!