Beat the butter until fluffy in a bowl, add the pinch of salt, the white pepper and the sour cream.
Work in the egg yolks, and beat until fluffy and light.
Whip the egg whites separately into a stiff foam, then gently fold them in together with the flour, into the yolk base.
Spread evenly onto a buttered, parchment-lined baking sheet.
Bake in a preheated oven at 190°C for 12-15 minutes, until set and lightly golden on top.
Meanwhile, sauté the onion in lard until translucent, add the sliced mushrooms, and fry over medium-high heat, stirring, for 8-10 minutes, until the mushrooms release and then cook off their liquid; season with salt and pepper.
Carefully turn the baked egg pancake out onto parchment paper or a kitchen towel.
Spread the mushroom filling over one half, then roll up the pancake.
Slice, and serve warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.