1. Heat half the oil in a large stock pot. Season the chicken with salt and pepper and fry in batches until the skin is browned. Leave to one side.
2. Heat the rest of the oil with the unsalted butter in the stock pot and fry the onion and garlic with a little salt until they start to soften. Stir in the plain flour for a minute or two, then pour in the stock. Keep stirring while it heats to make sure there are no lump of flour.
3. Once it is simmering, add the herbs, carrots, mushrooms and chicken along with any additional seasoning, cover then leave to simmer for 45 minutes.
4. To make the dumplings, rub the flour and butter together with your fingertips until you have a mixture that is a breadcrumb consistency. Stir in the herbs and some salt and pepper.
5. Drizzle the water over the top and use a rounded knife to form a dough. Shape into tablespoon-sized balls.
6. When the casserole has about 20 minutes left to stew, carefully place the dumplings on the surface and leave to cook.