To make the teriyaki sauce: combine the soy sauce, mirin, and sugar in a pan and slowly bring to a boil. Turn the heat down low and simmer for about 20 minutes, until it has thickened. Skim the surface of any scum if necessary and set aside
To make the tsukune: roughly chop the onion. Remove any stringy parts from the celery and chop it roughly
Put all the ingredients, except the basil, into a bowl and knead to combine well. Finally, chope the basil into tiny pieces and add to the mixture. It is important to add the basil at the last minute so it retains its color
Shape the mixture into rounds about 2 inches in diameter. Drizzle a little oil in a nonstick skillet and heat. When hot, add the tsukune and cook until nicely browned on both sides
Take the cooked tsukune, dip them in the teriyaki sauce while still hot, and sprinkle with shichimi togarashi/chili pepper and sansho/szehucan peppercorns according to your own preference. Serve with lemon wedges on the side