Squeeze the lemon and season with salt, pepper and sugar. Peel, halve and core the avocado and cut into large pieces. Peel the tomato and cut into large pieces along with the avocado. Peel the onion and add small pieces to the tomato and avocado. Puree everything. Add the puree to the lemon marinade, add the quark and stir everything together.
Tastes good with fondue, chopped vegetables and toasted bread.