Rinse tuna steaks and pat dry.
In a shallow bowl, combine the paprika, pepper flakes, the black and white sesame seeds, 1/2 teaspoon each of the salt, 1/2 teaspoon of pepper, lime zest, a bit of vegetable oil and grated lemongrass.
Add the tuna steaks to the bowl, coat them completely with the marinade. Marinate in the fridge for about 10 to 15 minutes.
Place the sesame seeds on a plate.
Remove the marinated tuna steaks from the marinade and coat each steak in sesame seeds on all sides. Pressing carefully the steaks to the plate of sesame seeds so they will stick.
Add oil to a cast-iron skillet over medium-high heat. Once the oil is hot, gently add the tuna steaks and sear for about 3 minutes on each side or a little longer to your desired doneness.
Remove from the heat and let rest for 3 to 5 minutes before serving.
Serving Suggestions: Serve with lime wedges and plenty of mayonnaise or a sauce of choice like aïoli. This dish goes well with a side of lettuce or other salad.