Preheat oven to 375 degrees. Prepare a 8-inch baking dish by spraying with baking spray. Gather your ingredients so that you have everything close at hand.
In a large bowl, whisk together the granulated sugar, cornstarch, ground cinnamon, and kosher salt. Add blueberries and lemon juice. Gently fold until blueberries are coated well. Spoon blueberries into the prepared baking pan.
Prepare the Crumble:
Using a food processor, grind old-fashioned oats until they become a fine powder. I like to grind a larger amount than what I need and then I'll store it for later use.
In a small bowl, combine the all-purpose flour, oat flour, brown sugar, and kosher salt. Add small pieces of butter and using your fingers, mix in until butter is fully incorporated.
Crumble the topping over the blueberries to cover the entire top surface.
Bake in preheated oven until lightly browned and sauce is bubbly, 30-40 minutes.
Serve with scoops of vanilla ice cream. Serves 4-6.
Notes
*Unless otherwise indicated, all butter on this recipe site is salted.