Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish or line it with parchment paper for easy removal.

In a large mixing bowl, combine the flour, oats, 1 cup of the sugar, cinnamon, and baking soda.

Add the cold cubed butter and cut it into the dry ingredients using a pastry cutter or two forks until the mixture resembles coarse crumbs.

Reserve about 2 cups of the crumb mixture for the topping.

Firmly press the remaining crumb mixture into the bottom of the prepared baking dish to create an even crust.

In a medium saucepan, combine the blueberries, remaining ½ cup sugar, cornstarch, and lemon juice.

Cook over medium heat, stirring constantly, until the mixture comes to a gentle boil and thickens, about 2–3 minutes.

Spread the warm blueberry filling evenly over the prepared crust.

Sprinkle the reserved crumble mixture evenly over the blueberry layer.

Bake for 25–30 minutes, or until the topping is lightly golden brown and the filling is bubbling around the edges.

Allow the Blueberry Crumble Bars to cool completely before slicing into squares.

Cut into 24 bars and serve.