Heat the olive oil in a heavy-based saucepan. Add the pancetta and cook over a medium heat until it is golden and has released its natural fats.
Add the onion, carrot, celery, leek, bay leaves, thyme and garlic to the pan and cook for 8-10 minutes.
Drain the ceps and squeeze dry, retaining the soaking liquor.
Chop the ceps finely and add to the cooked vegetables.
Stir in the lentils.
Strain the ceps' soaking liquor through muslin or a fine sieve straight into the lentil pan.
Add the wine and stock and bring to the boil.
Meanwhile, brown the sausages all over in half the butter. Cut them into 2.5cm (1in) pieces and add them to the lentils.
Cook for about 30 minutes until the lentils are tender.
Fold in the spinach and parsley and cook until wilted.
Fold in the remaining butter to enrich and season to taste.
Serve piping hot.
