In a pan, add your quartered mushrooms and water over medium high heat. Gentle cook the mushrooms letting the water evaporate fully so that the mushrooms soften.
Add olive oil, stir to coat, and then add spinch and pine nuts. Cook until spinach has wilted and pine nuts have browned. Turn heat off and remove from heat.
Add parmesan cheese to spinach and mushroom mix. Stir, and set aside.
Generously salt and pepper one side of the flank steak. Add your mushroom and spinach mix to the salted side.
Using a firm hand, roll the flank steak into a burrito and tie it together with some cooking twine. You should be left with something that looks like a burrito so that the edges are closed in on themselves. See IGTV video for directions.
Rub olive oil onto exterior of the meat, and salt and pepper the exterior as well.
Turn your grill on and let the temperature come to 450°F. When the grill is at temperature, add your meat and close the lid. Let cook for 8 minutes on that side, until a nice crust has been made.
After 8 minutes, flip steak over and sear on the other side.
If you are adding potatoes and veggies to the grill, now is the time to add them. Then close the lid and let sear for 8 minutes.
After you've seared the second side, reduce heat to 350°F and place meat on top rack of grill. Cook for an additional 35-45 minutes until internal temperature has reached 125°F or desired level of doneness. We like medium-rare.
Remove from heat, let rest for 5-10 minutes and slice to serve.