Add 2 tbsp sake and 1 tbsp mirin to a small sauce pan and heat on medium. Bring to boil and allow to bubble for 30 seconds.
Turn heat down to low and add 1 tsp sugar and ½ tbsp dashi stock. Mix over the heat until sugar is dissolved.
Once the sugar is dissolved, turn off the heat and add 1 tbsp soy sauce and 1 tsp sesame oil. Mix and remove from the stove. Leave to cool for about 10 minutes.
Using sharp knife, cut 150 g sashimi-grade tuna and 50 g sashimi-grade salmon into thin strips.
Finely dice the tuna and salmon using a rocking motion with your knife. As the tuna and salmon spreads out, scrape it back to the centre and repeat, cutting in different directions each time until a "mince" is formed.
Divide 2 portions cooked Japanese short-grain rice into portions and make a bed of 3 perilla (shiso) leaves and 2 tbsp shredded sushi nori seaweed "kizami nori".
Place the mashed tuna and salmon on top and sprinkle with 2 tbsp green onion and 1 tsp toasted sesame seeds. Arrange 30 g Japanese or Persian cucumber(s) on the side with 1 tsp wasabi paste to taste and pour the sauce over the top.
Mix well before eating and enjoy!