Combine all of the spices in a bowl. Rub 1 side of each tuna steak with some of the rub. Heat oil in a large pan over high heat. Place the tuna in the pan, rub-side down, and cook until golden brown and the spices have formed a crust, about 2 minutes. Turn over and continue cooking for 1 to 2 minutes for rare doneness. Drizzle each steak with some of the Mustard-Mint Sauce.