Heat the oven to 350°F (175°C). Line the pan with a parchment sling and lightly grease the exposed sides.
Combine flour, oats, brown sugar, baking powder, and salt. Cut in cold butter until the mixture forms uneven crumbs with no large butter cubes.
Reserve 1 cup of crumbs for the topping. Press the remainder firmly across the bottom of the pan and bake for 12 minutes.
Gently toss raspberries with filling sugar, cornstarch, and lemon juice just before using. Avoid crushing the berries into a puree.
Spoon the berry mixture across the warm base. Scatter reserved crumbs over the top, leaving small gaps where fruit can bubble through.
Bake for 30–38 minutes more, until topping is golden and the filling bubbles in the center, not only at the edges.
Cool completely in the pan for at least 90 minutes. The hot filling is loose and needs cooling time to thicken.
Lift out and cut into 16 bars. For sharper cuts, refrigerate the cooled slab briefly before slicing with a clean sharp knife.
