Preheat oven to 350°F. Lightly coat an 8-inch or 9-inch square pan or 1.5-quart baking dish with cooking spray or melted coconut oil.
In a medium bowl, toss together the peaches, blackberries, 2 tablespoons of coconut sugar, arrowroot powder, lemon juice, vanilla, cinnamon, and a pinch of salt.
Spoon the fruit filling and juice into the prepared baking dish.
Using the same medium bowl you used for the fruit filling(uncleaned, you want to use any fruit juice that was left behind), combined the rolled oats, oat flour, ⅓ cup coconut sugar, salt, cinnamon, and pecans (if using). Pour in the melted butter and stir until the butter has been fully incorporated into the oat mixture.
Sprinkle topping over fruit filling.
Bake for 40-45 minutes, until the topping is golden brown and the fruit filling is bubbling.
Let it sit for 10-15 minutes before serving.
Serve warm.
Cover and store leftovers in the refrigerator for up to 5 days.