Pour the soy sauce, mirin, water, and sugar into a small saucepan. Add the kombu and bring the mixture up to a boil. Reduce to a simmer and let cook for 15 minutes.
½ cup soy sauce, ½ cup mirin, ½ cup water, 1 tbsp granulated sugar , 1 3x3" piece kombu
When the 15 minutes are up, take the saucepan off the heat. Remove and discard the kombu and pour the mixture into a resealable container. Set aside to cool until at least lukewarm.
Lower the yolks into the cooled soy sauce mixture. Cover with a piece of paper towel. The paper towel will help keep the yolks immersed in the cure. Alternatively, you can flip the yolks every 12 hours to ensure they cure evenly. Leave the yolks to cure for 24-48 hours.
8 egg yolks
When the yolks are ready, remove them from the cure and place them in a resealable container. Pour the cure into a large skillet and place it over medium heat. Boil the cure for 10 minutes or until it reaches a syrupy consistency. Transfer to a jar and set aside to cool.
Rinse the sushi rice under cold water until the water runs clear. Pour the rice into a small saucepan or a rice cooker. Add the water and cook. For the stovetop version, bring the rice and water up to a boil, then reduce to a simmer. Cover and let cook for 15-18 minutes or until the rice is fluffy.
2 ¼ cups sushi rice, 2 ½ cups water
Transfer the rice to bowls. Drizzle the rice with the reduced soy sauce cure. Cover the rice with avocado slices and top the bowls with the Soy Sauce Cured Egg Yolks, microgreens, and a sprinkling of toragashi and sesame seeds.
4 avocados, ¼ cup radish microgreens , 1 tbsp toragashi , 1 tbsp toasted sesame seeds
