For the chicken, preheat the oven to 180C/350F/Gas 4.
Season the chicken with salt and freshly ground black pepper.
Heat a large casserole dish until hot, and the oil and brown the chicken on both sides. Remove the chicken from the pan and set aside.
Add the onion and fry for a minute, return the chicken to the pan then pour over the stout and stock.
Place in the oven, uncovered, for 1½ hours; basting every 30 minutes or so with the juices.
Remove from the oven, cover and set aside to rest for 15 minutes.
For the potato ragù, heat a large frying pan until hot, add the butter and potatoes and fry gently for 4-5 minutes, or until just softened.
Add the shallots, garlic, cabbage and thyme and fry for 3-4 minutes, or until the potatoes and cabbage are just coloured.
Add a couple of ladles of the cooking liquor from the chicken then bring to a simmer and cook for 2-3 minutes, or until the vegetables are tender.
Season with salt and freshly ground black pepper then stir in the parsley.
To serve, carve the chicken. Ladle the potato mixture into serving bowls then top with the chicken.