Preheat the oven to 180°, fan 160°C, gas 4.
In a large, flameproof casserole dish, heat the olive oil and fry the sausages, until browned. Remove the sausages from the dish and set aside.
Add the onion to the dish and fry until soft, then add the remaining vegetables to the casserole dish. Stir well and add the green lentils, pearl barley and 1 litre of stock, made using a chicken stock cube. Season well and cook over a high heat for 5 minutes.
Return the sausages to the casserole dish, transfer to the oven and cook for 1 hour, then serve.