Finely chop cilantro.
Using a microplane, grate the ginger and garlic into a small bowl.
Add cilantro, soy sauce, water, brown sugar and sesame oil into the bowl and mix well.
Season tuna steaks with salt and pepper, and coat the steaks with sesame seeds on both sides, pressing them down to stick to the fish.
Heat a medium stainless steel, or cast iron skillet, over medium high heat. Add oil to the heated skillet.
When the oil is shimmering hot, add tuna steaks and sear for 1-1.5 minutes. Flip and sear for another 1-1.5 minutes. (Feel free to cook a little longer to your desired doneness.)
At the last 30-seconds of cooking, add about half of the dressing into the skillet with tuna steaks. Cook for a few seconds, flip and sear for another couple seconds.
Remove the steaks onto a clean plate, slice and serve!