Pre-heat a griddle pan (or your grill) until hot and smoking.
Combine the mayonnaise, rosemary, garlic, salt and pepper and mix well.
Generously brush the mayonnaise mixture over both sides of the steak.
Place the steak onto the griddle pan. Cook the steak until done to your preference (I prefer rare-medium rare steak and always take my steak off a little before it's done as it will continue cooking while resting. As these steaks weren't very thick, I cooked them for 2 minutes per side).
Remove the steaks from the pan and allow to rest for 5-10 minutes.