With pastry blender, cut margarine and flour. Add sour cream and egg yolk. Form soft dough. Divide into two balls. Cover and refrigerate several hours or overnight.
Take dough out of refrigerator. Let sit a little to soften. It will still be very stiff. Roll out each dough very thin and into oblong shape. (I roll out between floured wax paper). NOTE: This will be making TWO strudels.
Mix all of filling ingredients together. Spread filling on dough coming to about an inch from the edge of the dough. Carefully roll into a jelly roll. Repeat with second dough ball.
Place both jelly rolls on parchment lined cookie sheet. Cut slits in dough at 1" intervals. Brush tops with egg whites. Bake in 350 oven for 40 minutes.
Remove from oven. Cool on pan. Make a thin confectioners glaze and drizzle over strudel. Slice and serve.