Sift both flours onto a board. Make a well in the center, and pour in a mixture of 1cup lukewarm water, the egg, vinegar and salt. Knead well. You should knead and beat this dough for as long as 10 minutes until you get a smooth, puckery dough. A good workout for the dough is very important because this is what makes it stretchable.
Make a round loaf out of the dough, put it on a floured board, spread it with some of the butter or lard, and cover it with a pot. Let it rest for 30 minutes.
Cover a large table with a clean white tablecloth and sprinkle it well with flour. Gently lower the dough onto the table. Carefully stretch it from all sides, using the backs of your hands. Form the dough into a longish shape. Keep going around the table. Put your hands under the dough, and keep stretching it without making a hole.
Cut off uneven edges with kitchen shears and sprinkle lukewarm water on dough. Knead it again into the same loaf shape. Spread it with butter or lard, and let it rest again, covered, for another 30 minutes.
Again start stretching it with the backs of your hands, and cut off edges with shears.
Leave the dough for 10 minutes to dry alittle so it won’t stick when you roll it. (Some Hungarian cooks speed this process by using an electric fan.) Preheat the oven to 375°F.
Sprinkle the dough with melted butter or lard. (By this time you will have prepared the filling of your choice.)
Place the filling mixture on one third of the dough sheet. Hold the edge of the tablecloth with both hands; gently lifting it upwards, roll the strudel dough into a roulade shape. Cut it according to the size of your baking pan. Place it in the greased pan and spread the top with more of the melted butter or lard.