Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

In a large bowl, whisk together the flour, rolled oats, sugar, baking powder, and salt for the crust/crumble mixture.

Cut in 1 cup of cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Reserve about 1 1/2 cups of this mixture for the topping, and press the remaining mixture evenly into the bottom of the prepared baking pan to form the base.

Bake the base layer for 15 minutes, or until lightly golden brown. While baking, mix the apricot jam and lemon juice together in a small bowl.

Remove the base from the oven. Carefully spread the apricot jam mixture evenly over the warm crust. Then, gently sprinkle the reserved crumb mixture over the top of the jam layer. If desired, cut the remaining 1/4 cup of cold butter into small flecks and dot over the top for extra richness.

Return the pan to the oven and bake for an additional 20-25 minutes, until the topping is golden brown and the filling is bubbly.

Let the crumble squares cool completely in the pan on a wire rack (at least 2 hours) before lifting out using the parchment paper and cutting into 16 squares.