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Turn on the grill. Preheat your grill to 450 degrees F.
Cook the pepper. Place the red bell pepper on the grill grates and roast for 3-4 minutes, turning frequently, until blackened across the surface. Transfer to a gallon zip top bag and close the seal. Set aside.
Flatten the steak. Lay a large sheet of plastic wrap on a large cutting board and top with the flank steak and then another sheet of plastic wrap. Using a meat mallet or the bottom of a heavy saucepan, pound the flank steak until it is an even 3/4 inch thickness all the way across. Remove and discard the top sheet of plastic wrap.
Season and begin stuffing. Season the top of the flank steak with the Beef Seasoning (or equal parts coarse salt and cracked black pepper). Lay the spinach leaves down in a single layer across the top of the flank steak.
Add the remaining toppings. Remove the roasted red bell pepper from the bag and use a paper towel to peel the blackened skin away from the flesh of the pepper. Use a knife to remove the core and seeds and slice the roasted pepper into strips. Layer the strips on top of the spinach and flank steak. Scatter the sliced red onion over the red peppers and top the flank steak with the shredded mozzarella cheese, staying about an inch away from the edges on all sides.
Roll the steak and secure. Use the bottom sheet of plastic wrap to lift and roll the flank steak, tucking and tightening the wrap as you roll. Secure with butcher's twine, tied every inch and a half across the length of the rolled flank steak. Trim any excess twine.
Grill! Place the rolled flank steak on the grill, close the lid, and cook, rotating the flank steak roll a quarter turn every five minutes. Close the lid after rotating each time. The roast should take approximately 20-25 minutes to reach an internal temperature of 130-135 degrees F (Medium Rare) depending on the thickness of your roll.
Rest, slice, and serve. Remove the flank steak from the grill to a cutting board and allow to rest for 10 minutes. Slice into 1.5 inch thick portions in between the butchers twine. Remember to remove each piece of twine before serving.