Mix sesame oil, soy sauce, ginger, garlic, onion, and lime juice together and coat the tuna steaks with the marinade, cover tightly, and refrigerate for at least an hour
Heat a non-stick skillet over medium high to high heat
When the pan is hot, remove the tuna steaks from the marinade and sear them for a minute to a minute and a half on each side
Remove from pan and slice into 1/4-inch thick slices