Heat olive oil in a heavy-based saucepan and cook pancetta until golden.
Add onion, carrot, celery, leek, bay leaves, thyme, and garlic and cook for 10 minutes.
Drain soaked ceps and squeeze dry, keeping the soaking liquid.
Finely chop the ceps and add them to the cooked vegetables.
Stir the washed lentils into the pan.
Strain the cep soaking liquid through a fine sieve into the lentil pan.
Add wine and stock to the pan and bring to a boil.
Brown the sausages in 25g of butter, cut into 2.5cm pieces, and add to the lentils.
Cook for 30 minutes until the lentils are tender.
Fold in the spinach and parsley and cook until wilted.
Fold in the remaining 25g of butter and season with salt and pepper.
Serve the dish immediately.