1. Preheat oven to 400 degrees Fahrenheit.
2. Lay flank steak out on a work surface and, using a meat mallet, pound to about 1/2″ thick. This will take several minutes, but start from the center and work your way out.
3. Lay the roasted red peppers and spinach evenly over the surface of the flank steak. Top with mozzarella cheese.
4. Starting at the long end of the steak closest to you, roll the steak up tightly until it is entirely rolled. Tie with kitchen twine so it will hold together while cooking.
5. Place flank steak on a baking sheet and drizzle with olive oil. Sprinkle with salt and pepper.
6. Place in the preheated oven and cook for 30 minutes for medium-rare, or 40 minutes for medium.
7. Remove steak from oven and allow it to rest for about 10 minutes before slicing and serving.
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