Butterfly Flank Steak: Lay the steak on a cutting board and pat dry. Slice horizontally through the thickest part, stopping ~½ inch from the edge. Open like a book. Cover with plastic wrap and gently pound with a meat tenderizer or mallet until even. Season both sides with salt and pepper.
Make the Filling: In a medium bowl, combine the egg, mozzarella, Greek yogurt, cream cheese, spinach, sun-dried tomatoes, panko, garlic, onion, parsley, lemon zest, lemon juice, and crushed red pepper flakes. Mix until well combined.
Fill and Roll: Spread the filling evenly over the tenderized flank steak, leaving about a 1-inch border around the edges. Starting from the long side, carefully roll the steak up into a tight log. Secure with kitchen twine or toothpicks to hold the shape.
Sear the Roll: Heat a large oven-safe skillet with a drizzle of olive oil over medium-high heat. Sear the steak roll on all sides until browned, about 2–3 minutes per side.
Bake: Transfer the skillet to a preheated oven at 375°F (190°C) and bake for 40-45 minutes, or until the steak reaches your desired doneness (135°F for medium-rare, 145°F for medium).
Rest & Slice: Remove from the oven, tent loosely with foil, and let rest for 10 minutes before slicing. Serve: Garnish with fresh rosemary and serve warm with garlic green beans