Preheat the oven to 350 degrees F (180 degrees C).
Heat the olive oil in a large casserole. Add lamb in batches, taking care not to overcrowd the pan. Season with salt and pepper and brown lightly over a high heat. Remove each batch from the casserole as it brown. Once all the lamb has been browned and removed from the casserole, add the onion, celery and garlic, reduce the heat and cook for 4-5 minutes until softened.
Return the lamb to the casserole. Pour in the wine and cook over high heat until it is dark brown and reduced by half. Add the chili flakes and juniper berries and cook, stirring, for just 10-15 seconds. Add the tomato paste, chicken stock, rosemary and about 250 mL of water, or enough to just cover.
Cover the casserole with a lid and bake in the oven for 45 minutes. Add the onions and potato and cook for another 45 minutes. Stir the parsley through just before serving.