Preheat oven to 350 degrees F.
In a large frying pan over medium-high heat, sautthe onions and mushrooms until tender; remove from heat an let cool.
In a large bowl, mix together the spinach, feta cheese, egg, bread crumbs, red pepper, and onion/mushroom mixture.
Sprinkle the flank steak with garlic powder, salt, and pepper. Spread and press the stuffing mixture onto the flank steak to approximately 1/2 to 1 inch from the border, all the way around.
Begin rolling up the flank steak with the edge closest to you and roll away from you (the steak should be rolled parallel to the grain so that once you cut into the steak, you are cutting against the grain). Once rolled, tie the flank steak roll up with cooking string every 2 inches or so. Sprinkle the roll with salt and pepper.
In an oven-proof skillet, heat approximately 1 tablespoon of vegetable oil on medium to medium-high heat. Once hot, place the roll in the pan and sear each side of the roll, approximately 2 to 3 minutes per side.
Once the stuffed flank steak roll is seared on all sides, place the skillet with the steak in your preheated oven and bake for approximately 20 to 40 minutes (depending on the size of the roll) until flank steak is medium rare or until the internal temperature reaches desired temperature on your meat thermometer (see below). NOTE: This steak is best cooked rare to medium rare.
Rare - 120F
Medium Rare - 125F
Medium - 130F
Once cooked, remove the stuffed flank steak from the over and place on a separate plate. Cover with aluminum foil and let rest for approximately 5 minutes before cutting.
While the steak is resting, place the same cooking skillet back on the stove on medium-high heat and add approximately 1 cup of beef broth. Continue cooking until the broth has reduced by 1/2. Turn off the heat and allow broth to cool (at least until the bubbling stops). Add tablespoons of butter, one at a time, and stir continuously until each tablespoon of butter is melted before adding more. Continue to add butter and stir until all the butter is incorporated.
Slice the rested flank steak into individual rolls and place on a serving platter. Drizzle the rolls with the prepared beef broth sauce and serve.
* Either have your butcher butterfly the flank steak or split the meat down the center so it lays flat and then break down the tough fibers by running it through his meat tenderizer. You can also tenderize the flank steak with a meat mallet.
Stuffed Flank Steak with Spinach and Feta Recipe: https://whatscookingamerica.net/KarenCalanchini/StuffedFlankSteak.htm