In a small bowl, combine the mustard, garlic, rosemary, lemon zest and juice and crushed red pepper. Schmear the steaks with this deliciousness, place in a shallow baking dish and let them hang out in the fridge at least 2 hours or overnight.
Preheat a grill to medium high. Season the steaks with salt.
When the grill is hot, brush it clean and oil the grates. When the flames have dissipated, place the steaks on a hot spot on the grill. Brush with the excess marinade and move the steaks away from the flame if there is a flare-up. Grill the steaks for 4 to 6 minutes on each side for medium rare. Remove from the grill and let rest for 5 to 10 minutes before slicing.