Fry the lamb fillet in 2 tablespoons of hot oil until it is firm all around. Then fry for about 10 minutes, turning several times. Season with salt and pepper. In the meantime, clean and wash the salad and cut into bite-sized pieces.
Wash the apple thoroughly, rub dry, quarter, core and chop. Peel and coarsely grate celery. Remove lamb fillet from the pan. Add vinegar and bring to the boil briefly. Chop the nuts.
Pluck the thyme leaves from the stalks. Add nuts and thyme to the vinaigrette, season with salt and pepper. Leave to cool. Mix the salad ingredients with the vinaigrette. Cut the lamb fillet into slices while still warm and arrange on the salad.