For the crust:
Butter, for coating the baking dish
1 cup gluten-free rolled oats
1 cup almond flour or almond meal
3/4 cup packed light brown sugar
1/2 cup sliced almonds
1/2 teaspoon fine salt
8 tablespoons (1 stick) unsalted butter, melted and cooled
For the 1/2 cup granulated sugar
1 tablespoon cornstarch
6 cups sliced fresh peaches (about 1 pound of slices from 8 large peaches)
1 tablespoon freshly squeezed lemon juice
Make the crust: Arrange a rack in the middle of the oven and heat to 350°F. Coat a 9×9-inch glass baking dish with butter.
Place the oats, almond flour or meal, brown sugar, almonds, and salt in a large bowl and stir until combined. Drizzle the butter over the oat mixture and stir to combine. Press about 1/2 of the oat mixture into the prepared baking dish.
Make the filling: Place the granulated sugar and cornstarch in a medium bowl and stir until lump-free. Add the peach slices and lemon juice and gently toss to coat. Transfer the peach mixture onto the prepared crust.
Crumble the remaining oat mixture onto the fruit, pressing it into clumps if desired. Bake until the oat topping is golden and the fruit is bubbling, about 1 hour. Cool for 20 minutes and serve with ice cream or vanilla butter sauce.