Cut daikon into ¾ inch (2 cm) rounds and cut in half if your daikon is more than 2-inches in diameter.
In a medium pot, add daikon and water just enough to cover it (about 2 cups/480 ml). In Japan, we use white rice water (from rinsing rice) to cook daikon. It’s believed that the rice bran from the rice water help reduce the bitterness.
Cook daikon on medium heat for 15 minutes, until a bamboo skewer goes through smoothly.
Drain the water and rinse under cold water to remove the rice bran. Quickly rinse the pot as we will use it for simmering.
While cooking daikon, peel the ginger and slice thinly. Save ⅔ of sliced ginger (for cooking) and julienne ⅓ of the ginger slices (for garnish).
Sprinkle salt on both sides of the yellowtail, including the skin. Let it stand for 5 minutes, and then cut in half, about 2-inch (5 cm) pieces.
Bring a medium pot of water to a boil. Once boiling, add sake. Then add in a few pieces of yellowtail. Cook until the surface of the fish turns white, about 10-15 seconds.
Take the fish out and place the pieces in iced water. Once cooled, about 10 seconds, remove the fish from the iced water and transfer them to a plate/tray lined with a paper towel.
In the medium pot (that you used for boiling daikon), combine water, sake, mirin, and soy sauce.
Add ginger slices and sugar and mix all together.
Add the yellowtail and bring it to a simmer. Using a fine-mesh sieve, skim the scum and foam if there is any (should not be much due to precooking).
Add daikon and coat well with the seasoned broth. Keep a bare simmer.
Once simmering, place an otoshibuta (drop lid) over daikon and keep a bare simmer for 15 minutes, flipping the daikon occasionally. Do not use a regular lid. We need some moisture to escape & the otoshibuta helps coat it with seasoned broth at all times.
15 minutes later, the dish is done cooking. You can serve it immediately; however, I highly recommend to let cool even for a few hours.
Remove from the heat and let cool, keeping the otoshibuta on so the surface won’t dry out. Don’t use a regular lid as the foods take a longer to cool. Once every 30 minutes or so, tilt the pot to coat the surface of the daikon with the seasoned broth.
Once completely cooled, you can refrigerate (especially overnight). Right before you serve, reheat until the dish is warm.
Serve the dish in a bowl with some broth. Top the fish with julienned ginger. Enjoy!