To the flour, add the egg, oil, vinegar, a pinch of salt, and enough lukewarm water to make an elastic, soft dough; rest for 20 minutes (or use ready-made strudel pastry).
Stretch the dough on a tea towel until thin and almost transparent.
Mix the ham with the egg yolks, butter and sour cream.
Whisk the 4 egg whites to stiff peaks, and gently fold into the ham mixture.
Spread the filling evenly over the strudel dough.
Drizzle with a little sour cream, then roll up tightly.
Place on a greased tray, and brush the top with melted lard or butter.
Bake in a 190°C preheated oven for 30-35 minutes, until nicely golden-red.
Serve immediately, cut into thick slices, while still warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.