Preheat the oven to 180C/350F/Gas 4.
Place the duck breast, skin-side down, into a cold ovenproof frying pan. Season with salt and freshly ground black pepper and heat over a medium heat until the duck begins to sizzle. Transfer the pan to the oven and roast for 7-8 minutes, or until the duck is cooked to your liking.
In a separate frying pan, heat the oil over a medium heat, add the onion and fry for 3-4 minutes, or until softened.
Add the mushrooms and continue to fry for a further 5-6 minutes.
Add the lentils, tomato purée, red wine and hot stock, bring to the boil, then simmer for 5-6 minutes until the sauce starts to reduce. Stir in two tablespoons of the chopped parsley.
To serve, spoon the lentil cassoulet onto a serving plate. Cut the duck into three pieces and arrange it on top of the 'cassoulet'. Garnish with the remaining parsley.
