Combine sesame oil, grated ginger, soy sauce, and black pepper with tuna loin in a 1-gallon ziploc bag for 1 to 2 hours.
Heat a cast-iron skillet on high heat with peanut oil.
Remove tuna from marinade and roll in sesame seeds and and salt until thoroughly coated.
Sear in skillet until sesame seeds are beginning to char, then turn.
Continue until the entire outside is seared, about 5 - 6 minutes.
Remove tuna from heat and allow it to rest for 3 - 4 minutes, then slice crosswise into steaks and serve.