Preheat oven to 425°F.
Butterfly the steak by laying it flat on a cutting board. Starting at the longest end, line your knife up parallel to the cutting board and slice through the center of the steak, stopping just before cutting it all the way through. Open up the flank steak up on the fold.
1 1/2 - 2 lb flank steak
Use a meat tenderizer or mallet to pound the steak to an even thickness, about 1/2 inch thick. Season with salt and pepper on both sides.
Salt and pepper to taste
In a large, oven-safe skillet, like a cast-iron skillet, heat 1-2 tablespoons of olive oil over medium heat. Add the mushrooms and cook, stirring occasionally, for 5-7 minutes until softened and slightly browned. Add the garlic and cook for another 2 minutes until fragrant. Stir in the spinach then quickly remove from the heat. Spread mushroom and spinach mixture evenly over the seasoned beef and sprinkle evenly with goat cheese.
2-3 tablespoons olive oil, 8 ounces mushrooms, 5 cloves garlic, 2 cups spinach, 2 ounces goat cheese
Roll the steak with the grain (the lines in the meat). Roll the steak tightly, tucking in any loose pieces as you roll.
Tuck int the ends of the steak as they're thinner and won't cook evenly. Use cooking twine to snugly loop and tie the twine along the steak so its tight enough to hold it together. You can also use several toothpicks if you don’t have twine.
Add 1 more tablespoon of olive oil to oven-safe skillet and heat over high heat. When the oil starts to ripple, carefully place the rolled and stuffed flank steak in the pan and sear for 2 minutes on each side, until deeply golden brown. Transfer the skillet to the oven and bake for 20-25 minutes or until a meat thermometer reads 135°F (medium rare) or reads to your desired doneness. Remove from the oven and allow to rest for 15 minutes before cutting. Cut into 2 inch medallions and serve.