Line a 9×9 pan with parchment paper and coat with nonstick spray. Set aside.
In a large bowl, whisk together flour, oars, brown sugar, cinnamon and salt. Add in the cold butter and using a pastry blender, cut the butter into the mixture until it resembles very coarse sand.
Divide the mixture in half and press half into the bottom of the prepared pan to form the crust. Set aside both the pan and bowl with remaining mixture.
Filling
In a large bowl, whisk together the granulated sugar and cornstarch. Add in the blueberries and coat in the sugar mixture. Add in the lemon juice and zest. Stir gently to combine.
Spread the blueberries on top of the crust. Sprinkle remaining crust mixture evenly on top of the blueberries and press lightly.
Refrigerate the pan for 30 minutes.
Preheat the oven to 350° F. Bake for 35 to 40 minutes until the fruit starts to bubble. Remove from oven and cool completely before cutting.
Notes
Use cold butter.
Let the bars cool completely. You can even chill them in the fridge to help them maintain their shape. If you eat them too warm, they will be messy.
Store bars in a sealed container for up to 4 days.
Storing Leftovers: Leftover blueberry crumble bars can be stored in an airtight container for 4 days outside of the fridge. Freezing Leftovers: Wrapped cooled blueberry bars in plastic wrap, then add to a freezer-safe bag or airtight container and freeze for up to 4 months. Let them thaw in the fridge overnight before serving. Source: If You Give a Blonde a Kitchen