Park a 6- to 6 1/2-quart pressure cooker pot on high heat. Place the onion, cut-side down, on the bottom of the cooker and sear until well-charred — 5 minutes.
Add the 2 halved carrots, 2 halved celery stalks, leek tops, and dried mushrooms to the pot along with the peppercorns and bay leaves. Follow with the chicken legs, thighs, wings, and finally the carcass. Add the water and use tongs or a large spoon to push as much of the chicken under water as possible.
Attach and lock the cooker’s lid (according to the manufacturer’s instructions, of course) and bring up to pressure over high heat. When the cooker’s whistle blows (it’s typically annoying and discordant...all the better to get your attention), back off on the heat until the cooker just barely hisses. This means you’re maintaining pressure but not just dumping a lot of steam into the room, which would waste moisture, not to mention energy. Cook for 30 minutes.
Remove the cooker from the heat and either vent the pressure by opening the pressure relief valve or simply place the covered vessel in the sink and pour running water over the lid for about a minute. At that point the pressure should have abated and the safety lock on the lid will open.
Strain the broth first through a colander set over a large bowl to remove large solids. Place a fine-mesh sieve lined with cheesecloth over the now-empty pressure cooker. Strain the stock a second time through the lined sieve into the cooker. You should have 8 to 9 cups of stock/broth. (It’s stock because bones were involved and broth because there was meat as well. So it’s...brock.) At this point you can stop, cool the “brock,” and then refrigerate for up to a week or freeze for pretty much forever.
To make the soup, add the shoyu, salt, and 1 teaspoon of the togarashi to the “brock.” Taste and add additional togarashi as desired. Add the chicken breasts to the pot, bring to a boil, then reduce the heat to medium-low and simmer until the chicken is just cooked through — 5 minutes. Transfer the chicken to a large bowl using tongs or a slotted spoon.
Add the remaining carrots and the parsnips to the broth and simmer for 3 minutes, then add the remaining celery and leeks. Continue simmering until the vegetables are just tender — 3 to 5 minutes. Kill the heat and season with lemon juice, if desired.
While the vegetables are cooking, shred the chicken breasts with forks or with a hand mixerThe mixer quickly produces a very fine shred. on low.
To serve, ladle the soup into bowls and top with the chicken, reserved celery leaves, and another grind of pepper.