1 Cook the lentils for 20 minutes in boiling water: they should remain al dente. Drain and refresh under running water.
2 Cut the apple into thin strips without peeling it. Peel and chop the shallots and celery stalks. Mix with lentils, apple, celery and nuts in a bowl. Add the diced Roquefort cheese.
3 For the Auvergne vinaigrette, mix the vinegar and mustard and season with salt and pepper. Emulsify with the two oils. Just drizzle the dressing over the salad and serve the rest on the side in a small container.