Put the lentils into a saucepan, cover with cold water and simmer for 20 minutes. Drain well and transfer to a bowl.
Meanwhile, cook the green beans in salted boiling water for 2–3 minutes, then plunge immediately into a bowl of ice-cold water. This is known as blanching. Drain and pat dry on kitchen paper.
Heat 1 tablespoon of the oil in a frying pan and fry the lamb fillets for 7 minutes, turning once. Transfer to a low oven, 150°C (300°F), Gas Mark 2, to rest for 5 minutes, reserving the juices in the pan.
Add the wine to the pan juices and boil until only about 1 tablespoon remains. Remove from the heat and whisk in the vinegar and the remaining oil and season to taste with salt and pepper.
Combine the lentils, beans, beetroot and mint in a bowl, add the dressing and toss to coat. Serve with the lamb.