Take 5-6 radishes still with their tops on, and slice the leaves and the radishes thinly, keeping both separate. In a small bowl, mix together 2 tbsp quark, 1 tbsp grated gruyère and the radish leaves, then season well. Spread 2 slices of toasted rye bread with butter, then spread the yogurt mixture on top. Garnish with the radish slices, a good grating of black pepper, a spritz of lemon juice and a few drops of olive oil.