Fry the sausages in butter and oil until they are browned.
Heat a knob of butter and two tablespoons of olive oil in a large pan.
Add the onion and cook for two minutes.
Add garlic, bacon, carrot, celery, leek, bay leaf, and cumin and cook for two minutes.
Add lentils and red wine and bring the mixture to a boil.
Simmer until the wine volume reduces by half, then add the stock.
Add the sausages and duck meat to the pan and cook for 25 to 30 minutes.
Stir in the sherry vinegar and fresh parsley.
Boil the potatoes in salted water for 12 to 15 minutes until tender.
Drain the potatoes and push them through a ricer into a saucepan.
Beat in the butter and cream over heat until the purée is smooth.
Season with salt and pepper and transfer to a piping bag.
Pipe the pomme purée into bowls and serve the cassoulet alongside.