Dissolve the yeast and sourdough started in half cup of lukewarm water. Mix rye and wheat flour in a bowl. Add the dissolved yeast, oil and 1/2 tsp salt. Use the dough hook of stand mixer to form a smooth dough. Cover and let rise for 40 minutes.
Divide the dough into four roughly equal portions. Dust with flour and roll out into very thin strips (15x20 cm) with a rolling pin. Cut the strips into 2 cm wide strips and place them next to each other on two baking sheets lined with baking paper. Remove excess flour with a brush. Brush the strips thinly with whisked egg white. Sprinkle with sesame, fennel, anise, coriander or flax seeds, or a mixture, as desired.
Bake in the preheated oven at 180 degrees (gas 2-3, convection 160 degrees) one after the other on the middle rack for about 20 minutes until the crisp breads are lightly browned and crispy. (The baking time varies depending on the thickness of the slices.)
Allow the baked crispbreads to cool completely on a wire rack. The bread tastes best freshly baked, but it can also be kept fresh in an airtight container.