Preheat the oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment, leaving an overhang for easy removal.

In a large bowl, cream the softened butter and sugar with a wooden spoon or electric mixer until pale and fluffy, about 2–3 minutes.

In a separate bowl, whisk together the flour, salt, and baking powder. Gradually add the dry mix to the butter-sugar until combined.

Fold in the rolled oats until the mixture looks sandy and holds together when pressed.

Reserve about half the mixture for the topping. Press the other half firmly and evenly into the bottom of the prepared pan to form the crust. Use the bottom of a measuring cup to compact it.

In another bowl, toss the blueberries with lemon juice and cornstarch until evenly coated. Spread this fruit mixture in an even layer over the crust.

Crumble the remaining oat mixture over the blueberries. It’s okay if there are large and small crumbs; the texture is lovely that way.

Bake for 30–35 minutes, until the top is golden and the filling bubbles at the edges. Rotate the pan halfway through baking for even color.

Remove from the oven and cool completely on a wire rack. Chill 30–60 minutes if you want cleaner slices, then lift from the pan and cut into bars.