Put the oil in a flameproof casserole over a high heat. When the oil is hot, add the chicken and onion, and fry for about 5 minutes until golden.
Stir in the remaining vegetables and cook for 2 minutes until starting to soften, then mix in the pearl barley, ale, stock and mustard. Season with salt and pepper and bring to the boil. Cover and transfer to a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour.
Make the dumplings almost at the end of the chicken stew's cooking time. Mix together the flour, suet and chives in a bowl and season with a little salt and pepper. Stir in enough of the measurement water, adding a little at a time, to mix to a soft, slightly sticky dough. Shape dessertspoons of the mixture into balls.
Remove the chicken stew from the oven, transfer to the hob and stir through. When the stock is boiling, carefully add the dumplings so that they sit on the surface, leaving space around each one. Cover and simmer for about 15 minutes until the dumplings are light and fluffy and the chicken is cooked through. (Keeping the lid on while cooking makes the dumplings rise well). Spoon into warm shallow bowls to serve.