1. Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the bars out later. This makes cleanup and serving much simpler.

2. In a large mixing bowl, cream together the softened butter, 1 cup granulated sugar, and vanilla extract until the mixture is light and fluffy. This should take about 2-3 minutes with an electric mixer. In a separate bowl, whisk together the 2 cups all-purpose flour, rolled oats, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a crumbly dough forms. The mixture will look like coarse crumbs.

3. Press about two-thirds of the crumble mixture evenly into the bottom of your prepared baking pan. Use the back of a spoon or your clean hands to create a firm, compact base. Make sure it extends to all edges of the pan.

4. In a medium bowl, gently toss the blueberries with the 1/2 cup granulated sugar, 2 tablespoons all-purpose flour, and optional lemon juice. The flour helps to thicken the filling as it bakes, preventing it from being too runny. The lemon juice brightens the flavor of the blueberries.

5. Pour the blueberry mixture evenly over the pressed crust in the baking pan. Spread it out gently with a spatula, being careful not to disturb the crust too much.

6. Crumble the remaining one-third of the oatmeal mixture evenly over the blueberry filling. Don’t press it down; you want it to remain loose and crumbly for that classic texture.

7. Place the pan in the preheated oven and bake for 40-45 minutes, or until the crumble topping is golden brown and the blueberry filling is bubbly around the edges. You might see some of the blueberry juices peeking through the topping.

8. Remove the pan from the oven and place it on a wire rack to cool completely. This is the most important step for clean slices! Cooling can take 1-2 hours. Once fully cooled, use the parchment paper overhang to lift the entire slab out of the pan and cut it into bars.