Season chicken thighs on all sides with a generous amount of salt and pepper. If possible, set chicken aside to "dry-brine" and let off some of the chill from the refrigerator for 30 minutes.
Heat about 2 tablespoons grapeseed oil in a large pot that will ideally hold all the chick in a single layer over medium heat. Place chicken in pot skin side down and brown, about 7 minutes. Turn chicken over and brown flesh side, about 5 minutes. Remove browned chicken to a plate; I actually use the turned over lid of the pot I'm using. Pour off or wipe out the chicken fat from the pot and discard.
While chicken is browning, make braising sauce: whisk together miso, chicken broth or water, and sake or mirin, sugar in a small bowl until miso is dissolved.
Add another 1 tablespoon of grapeseed oil to pot. Add sliced onions and cook until translucent, about 5 minutes. Add minced garlic and cook until garlic is fragrant and lightly toasted.
Return chicken back to pot, placing pieces skin side up, along with any juices that collected on the plate. Pour the braising sauce over the chicken in the pot.
Bring to a boil, then turn heat down to low, cover the pot with a slight tilt to vent, and simmer for 20 minutes until chicken is tender and cooked through. Every 10 minutes during cooking, check on the pot. Tilt the pot, grab some of the braising liquid, and pour over the top of the chicken so it doesn't dry out. If the braising liquid seems to be evaporating too much, add a little bit more chicken stock.
Add baby bok choy and mushrooms if using, pushing them in between the pieces of chicken. Simmer for additional 10 minutes until the bok choy is tender-crisp.
Taste, and adjust seasoning with additional salt, and/or pepper. Garnish with fresh sliced scallions and toasted sesame seeds.