Preheat the oven to 190°C.
In a small bowl, mix the yogurt and egg. In a large bowl, mix both types of flour, salt and baking soda and add the mixture of yogurt and egg. Stir with a spoon until all the ingredients are combined into a homogeneous mass. Place the dough on a lightly floured surface and knead for a few minutes to make it smooth and supple.
Form the dough into a ball and roll it out until it is about 3 cm thick. Sprinkle it with rye flour, place in a pan covered with baking paper, then score the top of the dough twice with a knife.
Bake in an oven preheated to 190°C for about 40 minutes, i.e. until the bread crust is firm and crispy.