Marinating the tuna for at least an hour and cooking it until it's just medium-rare ensures it doesn't end up dry. Be sure to spring for sushi-grade fish—the simplicity and deliciousness of this recipe make it worth the splurge.
In a shallow bowl, combine the sesame oil, coconut aminos, and lime juice. Add the tuna and coat well. Cover and let marinate in the fridge for one hour.
When ready to cook, remove the tuna from the marinade and shake off excess liquid. Coat in sesame seeds.
Heat a non-stick skillet over medium-high to high heat. When the pan is hot, sear the tuna for 2 to 3 minutes, flipping halfway, for medium rare. Sear longer if you prefer medium or well-done tuna.
Divide onto plates and enjoy immediately!