Using a sharp knife, slice the steak horizontally through the middle, until only about ½ inch is left. Fold it open and lay flat.
Cover the steak with parchment or wax paper then use a meat mallet or hefty rolling pin to pound it to an even thickness (¼-½ inch). This is a great time to get out some of that holiday stress!
Sprinkle the steak evenly with salt, pepper, and rosemary on both sides. Allow it to rest while you prepare the filling.
Heat the oil in a skillet over medium heat. Add the pepper flakes and the garlic into the oil. Saute until fragrant, about 30 seconds. Add the spinach and allow it to wilt, about 2-3 minutes. Sprinkle lightly with a little Kosher salt then remove the mixture from heat. Place the spinach mixture in a paper towel and squeeze gently (be careful, it's hot!) to release any excess moisture.
Add the spinach to a bowl and stir in chopped tomatoes, feta cheese, and lemon zest. Set aside.
Place the steak so the grain is running horizontally in front of you. Spread the feta mixture all over the top of the steak, leaving just a small edge. If you have edges that are torn or very thin, trim them off.
Roll the steak over the filling tightly from bottom to top. Wrap it with saran wrap and freeze for 15 minutes. This makes for easier slicing.
Once the steak has chilled, remove the saran wrap and preheat the oven to 375 degrees. Wrap a piece of butcher’s twine in the very center of the roll, then add more on either side, spacing each tie 2-2.5 inches apart. You want the twine to be in the center of each slice. Use a sharp knife to slice the steak. You should get 7-8 pieces.
Heat vegetable oil in a large oven safe skillet over high heat until it shimmers. Add the steak, filling side down, and brown for about 2 minutes. Flip and brown the other side, another 2 minutes. Pop the whole pan into the oven and bake another 5-9 minutes. Place a meat thermometer at the thickest part of the interior of the roll. Remove it around 145 degrees for a perfect medium.
Rest the steak for 10 minutes, then top with balsamic glaze before serving.