Season chicken with salt and pepper. Heat oil in a large Dutch oven over medium heat. Sear chicken, skin side down first, until golden brown on both sides, 10 to 12 minutes per side. Transfer chicken to a plate.
Stir garlic and farro into pot. Season with salt and pepper. Cook until garlic and farro begin to toast, 5 to 8 minutes. Add vinegar, yuzu kosho (if desired), and 4 cups water, scraping up any bits on bottom of pot. Bring to a simmer.
Return chicken to pot, skin side up, and cover. Reduce heat to medium-low; gently simmer until chicken is falling off the bone and farro is tender but not mushy, about 90 minutes. Stir in watercress before serving.