How to Make It
Heat grill to medium-high. Cut the steak in half horizontally (don’t cut all the way through) and open like a book.
Spread the tapenade over the steak; top with the spinach. Roll up; tie with twine at 2-inch intervals. Season with ½ teaspoon salt and ¼ teaspoon pepper.
Grill the steak, covered, turning occasionally, 15 to 18 minutes for medium-rare. Let rest for at least 5 minutes before slicing.
Meanwhile, in a large bowl, whisk together the oil, vinegar, mustard, and ¼ teaspoon each salt and pepper. Add the greens and toss. Serve with the steak.