Gather all the ingredients for making the Cold Brew Dashi and ramen broth. You can keep the ground chicken in the refrigerator for now.
In a large pot, combine 12½ cups water, ⅔ oz Kombu, ⅔ oz dried Shiitake, and ⅔ oz Iriko/Niboshi (boiled and dried anchovies). If you use several pieces of Kombu, count them so you know how many pieces to remove later.
Let it steep overnight (preferably) or at least 4 hours. Do not steep longer than 12 hours or the Kombu will turn slimy and the Dashi will be bitter. During summertime or hot weather, keep your pot in the refrigerator for food safety.
After steeping, reserve the specified amount of this dashi (½ cup, 120 ml) for making the Shio Tare later. Set it aside.
Thinly slice 1 knob ginger into about 3 slices. Cut and separate 1 Tokyo Negi (Naga Negi; long green onion) into four sections: 1) the white stalk; 2) the Y-shaped piece where the white stalk and light green leaves meet; 3) the light green leaves; and 4) the dark green tops. Reserve the outer layers of the white stalk for the Shiraga Negi (white scallion topping) and the stalk‘s light green core for the aroma oil (See Step 2 of “To Prepare the Toppings“ for how to remove the core). For making the soup, we‘ll use half of the dark green tops plus the Y-shaped piece (see center top of the photo, below right).
To the pot of 12 cups Cold Brew Dashi add 1½ lbs ground chicken, the ginger slices, half of the Negi‘s dark green tops, and the Y-shaped Negi piece.
Set the pot on the stove over medium heat. Using a wooden spatula, gently break apart the ground chicken while you heat up the stock. Do not leave any big chunks of chicken. Slowly bring the stock to a gentle boil to extract the flavor from the Kombu. Right before the liquid boils, remove all the Kombu pieces that you counted earlier and discard them.
Lower the heat and simmer the stock for 30 minutes. Remember to keep the stock gently simmering but not boiling. As we‘re using ground chicken, we do not need to skim the scum from the stock. During this time, you can start making the Chicken Chashu (see below).
After 30 minutes of simmering, add the thick shavings (Atsukezuri) of ⅔ oz Katsuobushi (dried bonito flakes) and gently simmer for another 10–15 minutes. If you are substituting with thin shavings (Hanakatsuo), add them to the simmering stock and cook for just 30 seconds, turn off the heat, and let the Katsuobushi steep for an additional 10 minutes.
Drain the stock in a fine-mesh strainer (especially if you‘re using thin shavings) over a large pot. I‘m using an 8-cup measuring cup to measure the total amount of soup. For 4 servings, I had 8½ cups of ramen soup after cooking. I then transferred the soup to the large pot. You can repurpose the spent ground chicken to make All-Purpose Miso Sauce, Soboro Don, Soboro Ankake, Keema Curry, Kimchi Soup, Kabocha Pork Stir-Fry, Mapo Tofu, and Mapo Eggplant.
Keep the pot of soup uncovered while prepping the other ingredients. The moisture will continue to evaporate, allowing the soup to develop more concentrated flavors.