Whisk together flours, sugar, and salt. If you don’t have oat flour on hand, finely grind rolled oats in food processor.
Using a pastry blender, cut butter into flour until it is edamame-sized. (You may be tempted to cut the butter into smaller pieces, however, as you add the water and continue to work the dough, the size of the butter will decrease.)
Add in the cold water. Use a pastry fork and work dough until it comes together. Gently knead with hands to incorporate any loose pieces. Do not overwork dough.
Shape dough into a disk, and wrap in plastic wrap. Refrigerate for at least an hour.
Make the Glaze
Meanwhile, make glaze. Add balsamic and maple syrup to a sauce pan. Bring to a boil. Reduce to a simmer until mixture has reduced in half. Remove from stove and allow to cool. Mixture will thicken.
Preheat oven to 375°. Line a baking sheet with parchment paper; set aside. Stir ricotta and maple syrup together. Set aside.
Roll out dough. Spread ricotta mixture around center of dough leaving a 1.5-inch clearing space around edges. Place sliced peaches on top. Fold crust over peaches. Brush crust with egg. Sprinkle with brown sugar.
Bake for 35 to 40 minutes. Serve warm or cool. Pour maple balsamic glaze on top just before serving. Best served same day.