Pour 3 tbsp Japanese soy sauce (koikuchi shoyu), 3 tbsp mirin and 3 tbsp sake into a small pan and bring it to a boil on medium-high heat. Boil for 1 min to burn off the alcohol in the mirin and sake.
Remove the pan from heat and swirl around a few times to release some heat. Pour the marinade into a wide heatproof container and add 5 g dried kelp (kombu). Leave to cool completely.
Cut 200 g sashimi grade salmon and 1 avocado into cubes. Once the marinade is cool enough to touch, place the salmon and avocado in a single layer then cover it with plastic wrap touching the surface.
Gently shake the container to encourage the marinade to rise towards the plastic wrap. If you have a lid, place it on the container and store it in the refrigerator for 30 mins. I recommend using this time to cook 2 portions cooked Japanese short-grain rice.
After 30 mins dish up the rice and roughly cut 1 Japanese cucumber. Scatter the salmon, avocado and cucumber over the rice.
Pour 1/2 tbsp of the leftover marinade over each bowl and sprinkle with toasted white sesame seeds, kizami nori (shredded nori) and chili threads. Add a blob of wasabi paste to taste.
Enjoy!
Notes
Look for "sashimi-grade" label - suitable for raw consumption. Any sashimi-grade fish or seafood works.
Substitute sake with dry sherry or white wine.
Allow marinade to cool completely before pouring over salmon to avoid cooking it.