Rosemary Garlic Stuffed Steak

Prepare a grill or grill pan over medium-high heat. Brush the grill with vegetable oil. Heat a large skillet over medium-high. Add olive oil, onion, and garlic, and cook, stirring occasionally, until tender and just starting to turn golden on the edges. Add spinach, one large handful at a time, stirring and letting the handfuls wilt before adding the next addition. When all the spinach is in the skillet, stir in ½ teaspoon of steak seasoning, vinegar, dried rosemary, and mustard. Remove from the heat. On a work surface, butterfly the steak. Starting at the thin, narrow end of the steak, slice it in half horizontally stopping about ½-inch from the opposite side, then open the steak up like a book so you have one big thin piece. Season the steak with another 1 ½ teaspoons of steak seasoning. Spoon the spinach mixture evenly over the top of the steak, leaving a 1-inch border on one long side. Tightly roll up the steak like a jelly roll, starting at the edge without the border and set seam-side down on the work surface.  Using butcher’s twine, tie the steak together around the circumference at roughly 1 ½-inch intervals. Season the steak all over with the remaining 1 ½ teaspoons steak seasoning.  Place the steak on the grill and cover the grill. Cook, covered, opening the grill to turn and brown all 4 sides then closing it again, until the steak is golden and a thermometer inserted into the center is 130° for medium rare, about 7 minutes per side. Remove the steak to a work surface and let rest 15 minutes. Then slice the steak into rounds, remove the twine, and serve.