Tvaroh, which is curds (you know, curds and whey like the nursery rhyme), is often used in sweets in Slovakia. Quark is the German name, also called farmers cheese. You could probably use ricotta cheese in a pinch. Make sure to use the full fat versions!
Mix all the ingredients together except raisins, making sure to mash the cheese well. Some recipes call for vanilla sugar, some call for lemon peel, some call for neither. Choose according to taste.
Drop in clumps along half of the dough and spread with your fingers. Sprinkle with raisins and melted butter. When you roll the strudel, after rolling the cheese part brush each round of dough with melted butter for extra flakiness. Continue recipe as above.