When kelp stock is infused with dried skipjack tuna flakes, or katsuobushi, it becomes another common Japanese stock, dashi. Dashi is a must-have ingredient that is the bast of everyone's favorite miso soup, udon and ramen broth, ponzu sauce, and many other recipes. To make dashi stock you must first make kelp stock by soaking kelp in water overnight.
People often call these flakes "bonito" fish flakes, but the fish that is actually used to make dried fish flakes is skipjack tuna. When you purchase died fish flakes look for those flakes that are glossy and lighter in color. —Hiroko Shimbo
Place the prepared kelp stock in a large pot over medium heat and heat up to 175° F. At this temperature you will begin to see little tiny bubbles building up and covering the side of the pot. Do not bring the stock to a boil. Add the fish flakes all at once, gently pushing the flakes down into the stock with a spatula. Wait for the stock to come to a gentle simmer, and then quickly turn off the heat. Leave the fish flakes in the stock for about 5 minutes.
Strain the stock through a moist paper towel-lined strainer, ensuring that none of the fish flakes pass into your stock, and reserve the fish flakes. Dashi should be very clean and clear; it will be much more flavorful than kelp stock. For best results, use it within three days, or store it in the freezer for later use.
After making your first dashi stock, be sure to make a second stock. Combine 6 cups kelp stock and the leftover katsuoboshi. Bring to a gentle boil, lower the heat to medium-low, and cook for 10 minutes. Add 1 cup of tightly packed katsuobushi and turn off the heat, then let sit for 5 minutes. Strain through a moist paper towel-lined strainer.