Cut 250 g yellowtail fillets into rough chunks and place them in a heatproof bowl. Sprinkle with ¼ tsp salt and set aside for later.
Peel 250 g daikon radish and cut it into thick half moons. Place it in a pot of water, bring to a boil over a medium heat and then lower to a simmer. Simmer for about 30 minutes and top up the water if necessary to keep the pieces submerged.
Prepare a bowl of ice-cold water, then pour freshly boiled water over the yellowtail.
After 10 seconds, transfer the yellowtail to the cold water to stop it from cooking. Drain the water and pat the surface of the yellowtail dry with kitchen paper.
Transfer the yellowtail to a clean pot and add 120 ml water, 60 ml sake, and 1 tbsp light brown sugar. Heat on medium and gently mix until the sugar dissolves.
When the liquid starts to boil, reduce the heat to medium-low and add 1 ½ tbsp soy sauce, 1 tbsp mirin, 1 dried red chili pepper and 5 g dried kelp (kombu).
Place a drop lid (or foil/baking paper cut to size) on top of the liquid and simmer for 15 minutes. While you wait, peel 10 g ginger root and cut it into fine matchsticks (these will be used to garnish).
Remove the drop lid and increase the heat to high for 2 minutes.
Divide the yellowtail and daikon between serving bowls and add a small amount of the cooking liquid. Garnish with ginger and enjoy!