Remove puff pastry from the refrigerator in advance to make it malleable. Prepare the stuffing: slice the onions lengthwise, then fry in a pan with butter, salt and thyme. Slice white sausage into half-moons and add to fried onions. Season stuffing with salt and freshly ground pepper. Spread pastry on the table, then cut it into 4 even stripes. Cut 2 pieces of pastry crosswise (from one edge to the other). Put the uncut pieces of pastry on a baking sheet lined with paper. Put stuffing onto pastry and sprinkle with crumbled curd cheese. Smear egg onto edges of the pastry. Put cut pieces of pastry on top and carefully glue the edges together. Smear egg onto top of the pastry. Bake in an oven preheated to 180*C for 15-20 minutes. Serve on a platter.