Step 1Place the sugar and mirin into a pot over medium heat and dissolve the sugar completely. Add the soy sauce and heat until a thermometer inserted reaches 158 degrees, then remove from heat. Cover the pot with a clean cloth (instead of a hard lid). Set aside and cool to room temperature. Store the liquid, covered, in the refrigerator. You can keep the base for a couple of months. This makes about 2 1/2 cups of hongaeshi.
Step 1Boil the water and add the dried bonito flakes. Lower the heat to a simmer and continue cooking for a minute. Strain the dashi liquid in a fine strainer.
Step 2Add the hongaeshi to the dashi and then heat it to 158 degrees. Add a dash of mirin and sake, if using. Cool the pot in an ice bath. This will keep in the refrigerator for 3 days. Serve on the side with soba.