In a bowl, mix all the ingredients to make the rub.
Coat each tuna steak with the rub, ensuring both sides are covered.
Heat a cast iron grill pan with oil (or fire up your outdoor grill). If using a cast iron grill pan, grill the tuna steaks for 2-4 minutes on each side, being careful not to overcook.
For an outdoor grill, preheat it to medium-high heat and grill the tuna steaks for the same amount of time.
Serve your perfectly grilled tuna steaks on a bed of fresh salad greens.