Sour Cream Apple Crumb Cake

Preheat oven to 350 degrees F (180 degrees C). Grease very well the bottom of a small rectangular pan (1,5 l capacity).
In a bowl combine all purpose flour, ½ cup einkorn flour and baking powder. Mix to blend.
In a small saucepan over medium heat combine butter and brown sugar. Mix until the butter is melted. Transfer the mixture to a large mixing bowl and add gradualy oil, egg, nutmeg, cinnamon and vanilla. Whisk until smooth.
Gently fold the ground walnuts and dry ingredients into the mixture.
Transfer about ⅔ of the batter into the prepared pan. Smooth it out with the spatula. Bake for about 20 minutes, or until a toothpick inserted in the centre comes out clean.
In the meantime add the rest of the einkorn flour to the batter to make a dough. If necessary, use a bit more all purpose flour as well.
Prepare the filling (see below) and spread it over the baked layer. Make the dough into crumbs and cover the filling with them. Bake for another 15 minutes and then switch the oven to top baking only, until the crumbs become golden brown.