For the rhubarb-apple strudel, peel the apples and cut into small pieces, peel the rhubarb and cut into slices. Mix remaining ingredients together with the apples and rhubarb except for the strudel sheets.
Place the 4 strudel sheets on top of each other on one side with the apple-rhubarb mixture, fold in the side ends 2 cm and roll up.
Brush with water and bake at 190 degrees for about 25 minutes in the preheated oven.