Mix the ginger, shallot, cilantro, jalapeño, soy sauce, and sesame oil.
Marinate tuna steaks in this mixture for 30 minutes to 8 hours, flipping once halfway through. (Add water to the marinade as needed to ensure it covers the fish.)
Heat the oil in a nonstick frying pan over medium-high heat until very hot.
Sear the tuna for 1-2 minutes, until browning is about ¼ inch thick and deep golden in color.
Flip tuna and sear on the opposite side for another 2 minutes until deeply golden brown on the outside and pink in the middle.
Slice the tuna against the grain and serve as desired.