Prepare the baking dish. Preheat your oven to 350 degrees F and lightly grease an 8x8-inch baking dish or 8 to 10-inch pie dish. Make the peach filling. In a large mixing bowl, combine peach slices, maple syrup, lemon juice, vanilla, cinnamon, nutmeg, cornstarch, and a pinch of salt. Gently toss until every peach slice is evenly coated.
Assemble the filling. Transfer the peaches into your prepared baking dish coated with nonstick spray and spread them into an even layer.
Mix the crumble. In a separate bowl, stir together the rolled oats, oat flour, chopped pecans, coconut sugar, cinnamon and salt. Pour in the melted butter, maple syrup, and vanilla. Stir until the mixture becomes evenly moistened and forms large crumbles.
Top the peaches. Sprinkle the topping mixture evenly over the peaches. Don’t pack it down, the loose crumbles create the crispiest topping.
Bake. Bake the peach crisp for 35 to 40 minutes, or until the peach filling is bubbling around the edges. The crisp topping will look golden brown and peaches should be fork tender.
Let it rest. Allow the crisp to cool for 10 to 15 minutes before serving. This gives the filling time to thicken. Serve warm with a scoop of ice cream, frozen yogurt, whipped cream, or a dollop of Greek yogurt.