(Day 1) Rinse and drain adzuki beans. Place them in a pot with plenty of water (not included in the recipe) and bring it to a boil. Once boiling, reduce the heat to medium and let it simmer for 5 minutes.
Drain the beans in a sieve and discard the cooking liquid to remove any bitterness.
Lightly rinse the pot, then add the beans and water (as specified in the recipe) again, and bring it to a boil. Once boiling, reduce the heat to low and let it simmer for about 20 to 40 minutes. The cooking time can vary depending on how long it has been since the beans were harvested. Taste one to make sure it has a "firm but edible" texture.
Remove the pot from the heat and let the beans cool down. After they have cooled, transfer them to a storage container and refrigerate. For the cooking liquid, reserve ¾ cup (180 ml for 4 servings) in a measuring cup and refrigerate it as well. Do not discard the remaining cooking liquid, as it will be used later.
Place a sieve over a bowl, add glutinous rice, and rinse it with water. Repeat this process several times, changing the water each time.
Soak the rinsed rice in the remaining cooking liquid in a pot in the refrigerator for at least 6 hours. If the rice isn't fully submerged in the cooking liquid, add more water to ensure it is completely covered.
(Day 2) Drain the soaked glutinous rice in a sieve and discard the soaking water.
Fill a steamer with plenty of water and bring it to a boil. Once the water is boiling, wet a steaming cloth, wring it out well, and line the steamer with it.
Evenly spread the glutinous rice on the steaming cloth inside the steamer. Fold the edges of the cloth inward to cover the rice, place the lid on the steamer, and steam it over high heat for 15 minutes.
Mix salt and sake into the reserved cooking liquid from the refrigerator.
Turn off the heat and carefully open the lid and steaming cloth to avoid burns from the steam. Add the adzuki beans from the storage container and mix them into the rice, then pour about one-third of the seasoned cooking liquid evenly over the mixture. Fold the steaming cloth back over the rice, cover the steamer, and steam it again over high heat for about 10 minutes.
Repeat the same step as the previous one, except for adding adzuki beans. If the water in the steamer is running low, add more hot water that has been boiled separately.
Repeat the previous step once more, but steam for 5 minutes this time. In other words, the total steaming times will be 15 minutes, 10 minutes, 10 minutes, and 5 minutes.
Take a bite of the rice, and if it is still firm, continue steaming for a few more minutes. Once it is done, move the rice to a different spot with the steaming cloth to prevent it from becoming soggy from the steam.
Serve in individual dishes and sprinkle with black sesame seeds and salt to taste. In Japan, to make sekihan visually appealing, it is often served on a flat dish or in a lacquered box.