Prep pan. Preheat the oven to 375°F and generously grease a 10 inch skillet, 9-inch pie pan, or 9x9 square baking dish.
Make ‘buttermilk’. In a small bowl or glass, mix together oat milk and lemon juice. Set aside.
Make filling. Cut the peaches in half, remove the pits, and cut into about ¼ inch thick slices. Place the peach slices and blueberries in your prepared baking dish. Top with brown sugar, tapioca starch, lemon zest, lemon juice, and a pinch of salt. Gently mix to combine and spread into a somewhat even layer.
Make the cobbler topping. In a large mixing bowl, combine the flour, sugar, baking powder, and salt. Work the butter pieces into the flour mixture with your fingers (or a pastry blender) until all the butter is covered in flour and you have pea-sized lumps (a few bigger chunks are fine).
Add wet ingredients. Fold in the buttermilk and vanilla extract until a wet biscuit dough forms.
Scoop. Use an ice cream scoop or large greased spoon to drop the biscuit dough into big dollops over the fruit mixture. Sprinkle a little turbinado sugar over the topping for extra crunch.
Bake. Bake for 40 to 50 minutes or until the fruit is bubbling and the topping is golden brown.
Serve. Allow to cool for a few minutes. Serve with a scoop of vanilla ice cream or dairy free whipped cream.