Gently fry the onion, carrot and celery in a pan with a bit of salt and pepper, for about 5-10 mins, or until soft and slightly brown, then place in the slow cooker.
Chop the sausages into large chunks, about 1inch thick, then place in the slow cooker along with the remaining ingredients.
Stir well and cook covered on high for 3 hours , or 6-8 hours on low. Add more water during cooking if necessary. The lentils should absorb the vast majority of the liquid, but it should not be a dry dish.
Serve on its own or with crusty buttered bread. Enjoy!