Roast the walnuts in a pan without fat, then chop them finely. Remove the core of the apple with a ball cutter. Cut the apple into thin slices, halve them again and sprinkle with some lemon juice.
Mix the quark and 2/3 of the nuts and spread two slices of bread with it. Wash the lettuce leaves and dab dry. Place on the quark, then cover with apple slices. Sprinkle bread with radish sprouts and remaining walnuts.