Step 1
Combine flour whole wheat flour yogurt milk oil sugar vanilla baking powder and salt in a bowl. Mix until dough holds together. Transfer to a lightly floured work surface and knead until a smooth dough forms. Add a little more milk if mixture is too dry. Set aside.
Step 2
Bring a large pot of water to a boil. Make an \X\ at the bottom of each peach and blanch in boiling water for 1 minute. Drain in a colander and immediately immerse in a bowl of ice water for several minutes to stop the cooking process. Drain.
Step 3
Remove the peach skins over a bowl to catch all the juice. Set bowl with juice aside. Remove stones and cut peaches into thick slices; place slices into a second bowl. Add brown sugar flour and cinnamon and gently toss so the peach slices do not break apart.
Step 4
Line a small baking sheet with parchment paper.
Step 5
Roll out the pastry dough into a 13x10-inch rectangle. Transfer onto the prepared baking sheet.
Step 6
Arrange peach slices like roof tiles in 2 long rows on the pastry leaving about 1 inch free on all 4 sides. Drizzle the reserved juice from the bowl over the peaches. Fold up the sides of the dough over the peach filling and gently press with your fingertips to secure. Place the baking sheet in the freezer for 15 to 20 minutes.
Step 7
Preheat the oven to 400 degrees F (200 degrees C).
Step 8
Remove baking sheet from the freezer. Brush the sides of the crostata with melted butter and sprinkle with demarara sugar.
Step 9
Bake in the preheated oven until golden brown 40 to 50 minutes.