Cut the sausages into bite-sized pieces, then set aside.
Finely chop the onion and garlic. Chop the mushrooms too.
Heat the oil in frying pan and gently sauté the onion for 5 minutes, then the garlic, mushroom and thyme leaves. Put a lid on and let it sweat for 5 minutes over low heat.
Add the vinegar and let it sizzle, then cook until it has evaporated. Add a drop of water, and the beans. Put a lid on and cook for 3 minutes over medium heat or until the beans are tender.
Stir in the lentils and mustard and warm through. Finally add the sausage pieces, put the lid on and warm through for a couple of minutes over low heat.
Serve warm.