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 some of the alcohol in the mirin and sake.
Remove the pan from the stove and swirl it 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 completely cool, 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 and touch the plastic wrap. If you have a lid, seal the container and store it in the refrigerator for 30 mins. I use this time to cook 2 portions cooked Japanese short-grain rice.
After 30 mins dish up the rice into serving bowls and roughly cut 1 Japanese cucumber. Scatter the salmon, avocado and cucumber over the rice.
Pour ½ tbsp of the leftover marinade over each bowl and sprinkle with toasted white sesame seeds and kizami nori (shredded nori). Add a blob of wasabi paste to taste. Enjoy!
Notes
Buy only fish labeled for raw use: Salmon with no such label is for cooking. Wild salmon and salmon sold to be cooked will not work in this bowl. In English-speaking stores, look for sashimi grade or sushi grade.Cool the marinade completely first: Pour it warm and the side it touches turns white and firm while the inside stays unmarinated. In a hurry, sit the container in an ice bath. Bring it down until it feels cold to the touch.A shallow marinade is correct: Do not make enough to cover. Lay wrap on the surface, shake the container gently, and this much reaches every side.Bring the rice down to skin-warm: Salmon fat melts well below body temperature. On freshly cooked rice the fat weeps out where it touches and the color dulls. On cold rice it stays firm and the sweetness never arrives.Do not marinate far past 30 minutes: Longer and the salt draws water out, the flesh goes dry and grainy, and the color drains. Lift the fish out at 30 minutes. Salt keeps going in the whole time.Eat it the same day: Salmon marinated raw does not carry over to the next day. Marinate only what you will finish.