Mixing: In a mixer bowl combine white whole wheat and oat flours, salt, baking powder, and yogurt. Mix until it is crumbly. In a separate bowl, blend the starter and egg together. Add to the pastry crumbs in the mixer bowl. Turn mixer on and off a few times until the dough is mixed. Do not mix too much or the dough will be tough when baked. Transfer the dough to a piece of plastic wrap. Wrap to seal the dough.

Cold Ferment: place dough in the fridge for 24 hours.

To assemble: Preheat oven to 350 degrees F. Core apples and cut in half. Do not peel. Thinly slice each apple half. Place apple slices in a microwave safe bowl. Add water to cover the apple slices. Add lemon juice. Carefully stir the apple slices in the water. Microwave the slices for 3-4 minutes or until they are softened enough to bend. Drain and set aside. Roll out dough into a 10 inch by 16 inch rectangle. Cut into 8 pieces 2 inches by 10 inches. Spread each piece of dough with apple butter. Layer apple slices, overlapping on the long edge of the dough, making sure that the top of the slices are above the dough. Roll the dough/apple slices up. Place each tart in a tart pan, large muffin tin, or on a cookie sheet. Bake for 30 minutes or until the dough is fully cooked. Remove from the oven. Sprinkle with cinnamon or confectioner sugar before serving. 8 apple tarts