Peel the upper to middle section of the daikon, cut it into bite-sized chunks, and blend it with the water in a blender or food processor until smooth.
Transfer the puree to a fine-mesh strainer and let it drain naturally for about 5 minutes. Don’t press it down, as a little moisture helps create the right texture.
Meanwhile, whisk together the soy sauce, mirin, sake, and sugar until the sugar dissolves.
Add the drained daikon to a bowl with the potato starch and mix well. The mixture will seem quite loose, but it will still hold its shape once formed.
Scoop about a ping-pong-ball-sized portion of the mixture and gently flatten it into a small patty.
Add the sesame oil to a cold nonstick skillet, arrange the patties in a single layer, then turn the heat to medium. Cook for 2–3 minutes per side until golden brown.
Turn off the heat, pour in the prepared sauce, then return the pan to medium-low heat. Let the sauce gently simmer for 3-4 minutes, and as it thickens, carefully flip the patties so they’re evenly coated.
Transfer to a serving plate and sprinkle with the sesame seeds before serving.
