Peel and finely chop the onion. Wash and drain the lentils. Bring the lentils and approx. 300 ml water to the boil and simmer at low heat for 10-15 minutes (the liquid should be completely boiled). Heat 1 tsp. oil in a frying pan. Sauté the onion cubes in it for 2-3 minutes until translucent, add to the lentils. Leave to cool
In the meantime, wash the cucumber and radish, rub dry, halve and cut both into fine sticks. Stir 300 g quark with 1 teaspoon sugar until smooth. Mix in cucumber and radish. Season to taste with salt and pepper
Clean and wash the watercress and let it drain in a sieve. Wash, clean and slice the tomatoes. Mix vinegar in a bowl with 1 teaspoon sugar. Fold in 3 tablespoons of oil in a thin stream. Season to taste with salt and pepper
Add egg yolks, 200 g quark and flour to the lentils, season with salt and pepper. Heat 1 tablespoon of oil in a frying pan and add 3 heaped tablespoons of dough as 4 round cakes. Fry the roasts on both sides for 3-4 minutes each until golden brown. Finally add 1 tsp. butter. Keep warm. Bake 8 more roastings from the remaining dough
Arrange lentil roasts on plates together with the quark dip. Sprinkle with coarse pepper from the mill. Mix tomatoes and watercress with the dressing and arrange in bowls