Butterfly the flank steak. Using a sharp knife, butterfly the flank 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 two separate pieces. Open up the steak like a book, pound it to a 1/2-inch thickness, and set it aside.
Make the stuffing mixture. In a mixing bowl, combine the chopped spinach, cheese, sun-dried tomatoes, garlic, basil, and black pepper; mix until well incorporated.
Fill and roll. 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.
Brush with the tomato sauce. Spread the rest of the tomato sauce over the top and around the edges of the steak.
Roast. Transfer the steak roll to a baking dish and bake for 35 to 40 minutes or until the steak's internal temperature registers at 130˚F.
Rest. Remove it from the oven and transfer the steak to a cutting board. Let it rest for 10 minutes before removing the kitchen twine.
Cut and serve. Remove the 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.
Notes
Sharp Knife: A sharp knife makes it easier to neatly butterfly the steak before cooking—and after it’s done, use that same knife to slice against the grain for the most tender, juicy results.
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.
Grilling option: 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 the 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.