Cut each chicken thigh in half. Heat the oil in a flameproof casserole, add the chicken and onion and fry over a high heat until golden. Stir in the remaining vegetables and cook for 2 minutes, then mix in the barley, ale, stock and mustard. Season and bring to the boil. Cover and transfer to a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour.
When the chicken has finished cooking, make the dumplings by mixing the flour, suet, chives and some seasoning in a bowl. Stir in enough water to mix to a soft, slightly sticky dough, then shape dessertspoons of the mixture into balls.
Stir the chicken stew and transfer to the hob. When the stock is boiling, add the dumplings, cover and simmer for about 15 minutes or until the dumplings are light and fluffy. Spoon into shallow bowls to serve.