Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch baking pan.

In a food processor, pulse together the flour, sugar, and salt.

Add the cold butter and pulse until the mixture resembles coarse crumbs.

Press the mixture into the prepared baking pan.

Bake for 15-20 minutes, or until lightly golden.

In a medium saucepan, combine the raspberries, sugar, cornstarch, and lemon juice.

Cook over medium heat, stirring constantly, until the mixture thickens and bubbles.

Remove from heat and let cool slightly.

In a bowl, combine the flour and brown sugar.

Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.

Pour the raspberry filling over the baked shortbread crust.

Sprinkle the crumble topping evenly over the filling.

Bake for 20-25 minutes, or until the topping is golden brown.

Let cool completely before cutting into bars.