Place the frozen sheets of puff pastry together and defrost. Peel and chop the onion. Rinse the parsley, dry it and chop it. Sauté both together in sunflower oil or other vegetable oil.
Coarsely grate the pumpkin on a vegetable grater, wrap it in a thin tea towel or muslin cloth and squeeze it well. Add to the onions.
Beat the low-fat quark together with the butter, sour cream and eggs with a mixer until creamy. Gradually fold in the flour. Season with caraway seeds, salt, pepper and nutmeg (freshly grated). Add the onion-pumpkin mixture and mix everything together.
Fold the thawed puff pastry sheets over each other, lapping to form a large dough sheet, pressing the edges together well.
Spread the pumpkin mixture evenly over the pastry sheet and roll it up lengthwise. Place the strudel on a baking tray lined with parchment paper, brush with the egg yolk and bake in a heated oven at 180 °C for about 40 minutes.