Coarsely chop the walnuts. Onions peel, halve and in fine columns cut. Cut bacon into fine strips. Heat 1 tablespoon of oil in a small pan. Fry the bacon until crispy.
Pour vinegar and approx. 3 tablespoons of water. Add onions and walnuts, simmer briefly. Season with salt, pepper and 1 pinch of sugar. Pour into a bowl. Fold in 3 tbsp. oil. Cool down.
Heat butter in a large frying pan. Fry the Maultaschen in this pan for about 5 minutes, turning them golden brown. Clean and wash the radicchio and lamb's lettuce, if necessary pluck smaller and drain.
Cut Maultaschen in half. Mix salads and nut vinaigrette and serve with the Maultaschen.