Pre-heat the oven to 200°C.
Brush the fillet of lamb with the sunflower oil then season well. Heat a heatproof frying pan over a moderate heat until hot. Sear the lamb all over until golden brown in colour, then add the slices of lemon to the pan and transfer to the oven for 8 minutes.
Meanwhile, bring the lamb stock to a simmer in a large saucepan. Blanche the broad beans in a large saucepan of boiling, salted water for 1 minute. Drain and add to the lamb stock, then adjust the seasoning to taste with salt, pepper and lemon juice. Remove the lamb and allow to rest covered with aluminium foil for 5 minutes.
Spoon the broad beans and a little of the lamb stock onto serving plates. Slice some of the lamb fillet and serve on top of the broad beans, along with a few slices of lemon. Garnish with a sprig of parsley before serving.