Place the kombu, soy sauce, mirin, sake and sugar in a saucepan over a medium heat and bring to a simmer, then remove from the heat and add the bonito flakes. When slightly cooled add the lemon juice and let sit for one hour. Pass through a fine sieve and keep in the fridge until needed.

Mash two of the avocados with the sesame oil, sliced spring onions and wasabi, season with a little salt as the ponzu sauce will add salt when the dish is complete.

Dice the tuna and remaining avocados into 2cm pieces.