1. Place steak on a work surface. Holding a sharp carving knife parallel to the work surface and positioned along 1 of the longer sides, cut steak almost in half horizontally, so it opens like a book.
2. Spread steak open with "spine of book" parallel to you. Sprinkle panko mixture evenly over steak, leaving a 1-inch border along the farthest edge. Arrange spinach on top of panko mixture.
3. Beginning with the closest edge, roll steak up, gently pressing down on spinach.
4. Rotate steak roll so it's vertical to you and tie crosswise at 1-inch intervals with kitchen string. Place steak on rack in a prepared baking pan and bake as per recipe instructions.
5. Transfer cooked steak roll to a cutting board, loosely cover with foil and let rest for 10 minutes.
6. Remove foil and string and carefully cut steak crosswise into 1/2-inch slices.