Roll out the puff pastry and spread the tuna on top. Now spread the grated cheese on top and carefully roll the dough into a roll. Cut the roll with a sharp knife into 1 - 2 cm thick slices and place them on a baking sheet.
Preheat the oven to 200 ° C and then bake the pieces for about 20 minutes on top / bottom heat.
For the tzatziki, wash and slice the cucumber, finely chop the garlic cloves and mix both with quark, yoghurt, crème fraîche and thyme. Now season the tzatziki with salt and pepper and place on the warm tuna slices.