1. In a cooking pot, add the uncooked Japanese rice and soak it in water for about 20 min.
2.meanwhile make the sushi vinegar
Add all the sushi seasoning ingredients (Vinegar, sugar and salt). Cook over medium-high heat and melt the sugar and salt. Remove from the heat when it is almost boiling. Set aside.
3. After soaking the rice, drain well and transfer to a cooking pot. Add 400ml water. Cover and cook over high heat, then bring it to a boil.
4. Once boiling, reduce the heat to low. Mix once then cover again and simmer for 10 minutes.
5.meanwhile prepare the fish
Wash the fish and drain with a kitchen towel. Place on a tray or plate. Sprinkle kosher salt (1 tsp) and coat well on both sides (make sure no bones are left in the middle). Leave 10 to 15 minutes.
6. After 10 min simmering, remove the rice from the heat. Leave another 10 minutes but do not lift the cover yet. Then transfer into a moistened heat-safe bowl (or sushi "oke"). Pour the sushi vinegar on it in one go. Slice the rice and let it cool down. Cover and set aside.
7. Now wipe the fish's excess salt with a kitchen towel. Add oil (1 tsp) on a frying-pan and heat over medium heat. Wipe any excess oil. Place the filets skin side down. Cook for about 4 minutes. Then flip them gently and cook another 3 to 4 minutes. Set aside.
8. _ Prepare the sauce for the fish_
Add all the sauce for fish ingredients into a cooking pot. Cook over medium heat and bring it to a boil, then reduce the heat to low and simmer for about 1 min. Set aside.
9. _ Putting the sushi together_
Place a nori (shiny side down) on your sushi rolling mat. Spread 1/4 sushi rice evenly, leaving a space of about 2cm uncovered along the edge. Cover the rice with saran wrap and flip. Roll up tightly, while removing the saran wrap at the same time.
10. Once rolled up, sprinkle some scallion and place the fish. Brush with the sauce. Wrap with saran wrap tightly. Set aside and repeat for the 3 other rolls.
11. Cut the sushi roll and clean your knife each time you finish a slice. Note : Keep the saran wrap when you cut the sushi. Remove it when the slicing is done.
12. Place the sliced sushi and add more sauce if you like.