Mix the soy sauce, mirin and sesame oil and season with sezchuan pepper to taste. Place the piece of tuna in the marinade for at least 15 minutes. Halve the avocado and remove the stone, leaving the skin on.
Place the teppanyaki plate on the (round) Shichirin and heat it up well. Briefly fry the tuna on both sides until it has a nice crust and leave to rest next to the fire. Fry the avocado on the flesh side until it colours. Then remove the avocado from the teppanyaki plate and, using a spoon, hollow out the flesh, cut into thin slices and season with coarse salt.
Now also slice the tuna and serve on the shiso sheet alternating slices of tuna and avocado. Serve the tuna marinade separately as a sauce.