Coat the short sides of the tuna with sesame seeds.
Heat the coconut oil in a skillet until it melts and is hot (but not smoking). Place the tuna in the skillet and let it cook about 60-90 seconds per side. Immediately remove from the pan and place on a plate.
In a small sauce pan combine the tamari, ginger, coconut sugar, vinegar, and wasabi. Mix well and heat on a simmer until it starts to gently bubble. Add in the butter, mix as it melts.
Immediately spoon the sauce over the seared tuna, slice against the grain, and serve.