Preheat the oven to 350°F.

Coat a 9x9-inch square baking pan with vegetable oil spray.

In a large ceramic bowl, whisk together the flour, oats, both sugars, baking powder, salt, and lemon zest.

Pour the melted butter over the dry ingredients.

Utilizing a fork or clean hands, stir the mixture until it is coarse and crumbly.

The dough should hold together when pressed but remain distinctively pebbled.

In a separate small bowl, stir together the blackberry jam and cornstarch.

Ensure the cornstarch is fully incorporated to prevent weeping and ensure a stable, set center after baking.

Press exactly half of the oat dough firmly into the bottom of the prepared pan to create a solid base.

Spread the stabilized jam mixture evenly over the top.

Crumble the remaining dough over the surface of the jam, allowing small pockets of the fruit to peek through.

Bake for 35 to 40 minutes until the top is deep golden brown and the jam is bubbling at the edges.

Cool completely in the pan on a wire rack before cutting into 2-inch squares to ensure the structural integrity of the bars.