Fry the sausages in a little fat over a low heat. They will take up to 40 minutes, turning from time to time.
Meanwhile get the rest of the ingredients ready. Rinse the lentils in a sieve.
Fry the pancetta in a deep saucepan until the fat runs, then add the onion and fry for 4-5 minutes until it starts to soften. Add the mushrooms and garlic, stir then cover with a lid and cook for 5 minutes.
Add the lentils and enough boiling water to cover them by about an inch. Cook over a moderate heat for 20 minutes or so until tender but not too soft.
Season with salt and pepper then stir in the chopped parsley. When the sausages are done increase the heat under the lentils to evaporate most, but not all, of the liquid.
Stir in a few drops of the balsamic vinegar, tasting as you go. Slice the sausages thickly then serve the lentils on warmed plates with the sliced sausages on top.