In a small bowl, whisk together mustard, steak sauce, olive oil, vinegar and rosemary.
Spread marinade on roast, cover with plastic wrap, and refrigerate for 4 to 12 hours.
Allow the roast to stand at room temperature for 20 to 30 minutes before grilling.
Lightly season both sides of the roast with salt.
Sear over direct medium heat for about 10 minutes, turning once halfway through searing time.
Continue grilling over indirect medium heat until internal temperature reaches 135º F. for medium rare, 20 to 30 minutes more.
Remove from grill and allow to rest for about 5 minutes.
Cut across the grain into thin slices.