If you weren't able to find a butterflied flank steak at the store (or the butcher wasn't able to do it for you,) you might need to prepare your flank steak. You are aim for 1/4- 1/2 inch thickness. Lining a very sharp knife parallel with your cutting board and slice through the center of the steak, stopping just short of cutting all the way through. You should be able to open it up like a book and the surface area of the meat will have doubled. Use a meat mallet to even it out even further. The thinner the steak, the easier it will be to roll and the more fixings you can afford to stuff in there.
Preheat oven to 375 degrees. Coat the bottom of a 9x13 inch glass baking dish with cooking spray.
Heat a small skillet prepped with cooking spray over medium heat. Add mushrooms, garlic, parsley, sweet onion, cooking until reduced in volume by half. Remove and allow to cool slightly.
Lay your butterflied flank steak out on a cutting board and tenderize lightly with a meat mallet. Season well with fine sea salt and freshly ground black pepper. Evenly distribute mushroom mixture, leaving about 1/2 inch along the edges. Evenly distribute feta cheese.
Roll steak along the grain, tightly roll up flank steak into a roll. Fasten with cooking twine. Twine should be tight enough to hold it in a roll, but not so tight that your stuffing it leaking out.
Rub olive oil on the exterior of your rolled flank steak. Season exterior with fine sea salt and pepper. Transfer to prepared 9x13 baking dish.
Pour beef consume into the bottom of the baking dish.
Bake for 35-45 minutes, depending on how well you like your steak.
Remove and allow to set for 5-10 minutes before slicing.
If you've tried this recipe, come back and let us know how it was!