Using a sharp knife, butterfly the steak by placing your hand on top of the meat and cutting horizontally into the steak until just about 1 inch before you would cut the meat into 2 separate pieces. Open up the steak like a book, pound it to a 1/2-inch thickness, and set it aside.
In a mixing bowl, combine chopped spinach, cheese, sun-dried tomatoes, garlic, basil, and fresh ground pepper; mix until well incorporated.
Spread 1/3 cup of tomato sauce over the steak and top it with the prepared spinach mixture.
Tightly roll up the flank steak and secure it with kitchen twine. Brush the entire steak with olive oil and season with salt and pepper.
Spread the rest of the tomato sauce over the top and around the edges of the steak.
Transfer to a baking dish and bake for 35 to 40 minutes or until the steak's internal temperature registers at 130˚F.
Remove from oven and transfer the steak to a cutting board. Let it rest for 10 minutes before removing the kitchen twine.
Remove kitchen twine and slice the stuffed steak into 1/4 inch rounds.
Spoon tomato sauce from the baking dish over the rounds, top with fresh parsley, and serve.
Sharp Knife: Ensuring your knife is sharp will help you butterfly the steak more easily and neatly.
Pound It, if necessary, after butterflying; if the steak is still thick, gently pound it to about half-an-inch thickness using a meat mallet. Be careful not to pound it excessively, as it might rip.
Prep in Advance: You can prepare the beef and keep it in the fridge for up to 8 hours before cooking.
Add Nuts: Mix in some crushed walnuts into the filling for added crunch.
Layer with Prosciutto or Bacon: Place thinly sliced prosciutto or bacon on the steak before adding the spinach and feta filling for an extra layer of flavor.
Grilling option: If you'd like to make this on the grill, preheat your grill to 450˚F with the lid closed. Transfer the stuffed steak to the grill and cook for 10 minutes per side or until the steak's internal temperature reaches 130˚F. Remove from grill and let rest for 10 minutes before cutting.
To Freeze: Cut the cooked and cooled steak into pinwheels and wrap each pinwheel in plastic wrap. Transfer them to freezer bags and freeze for up to 6 months.
Nutritional info is an estimate and provided as courtesy. Values may vary according to the ingredients and tools used. Please use your preferred nutritional calculator for more detailed info.