Preheat your oven to 350°F. In a large bowl, whisk together the flour, baking powder, and salt to distribute the leavening agent evenly—this prevents dense pockets in your bars. Stir in the oats and brown sugar, mixing until combined. Pour in the cooled melted butter and vanilla extract, then mix with a fork or your fingertips until the mixture resembles coarse breadcrumbs with some pea-sized clumps remaining. I like to let the butter cool slightly before adding it so the oats don't clump together, which keeps the texture light and crispy.

Measure out 2/3 of the oat crumble mixture (about 2 1/4 cups) and press it firmly and evenly into a 13x9 inch baking pan, creating a compact base layer. In a small bowl, whisk the apricot jam with the lemon juice to loosen it slightly and add brightness—this helps it spread more easily and prevents thick clumps. Spread the jam mixture evenly over the base layer, leaving a small border at the edges. Sprinkle the remaining 1/3 of the crumble mixture over the jam, pressing it down gently with your fingertips so it adheres slightly but stays in a crumbly texture.

Bake the bars in your preheated 350°F oven for 35-40 minutes, until the crumble topping is golden brown and the filling is bubbling slightly at the edges. The jam should be set but still soft when you remove the pan from the oven. Let the bars cool completely in the pan on a wire rack—this is important for the jam to fully set and the bars to hold together when cut. I find waiting at least an hour makes cutting cleaner, though you can speed this up by placing the pan in the refrigerator for 20-30 minutes. Once cooled, cut into 12-16 bars depending on your preferred size.