I am posting the recipe as is, but I think adding about a cup of old-fashioned oats (not quick cooking) would be a good addition to the topping. I haven't done that in this particular crumble, but it really does add a lot to a crumble topping.
Now, the recipe:
Raspberry Squares with Brown Sugar Crumb Topping
Stonewall Kitchen Cookbook (Jonathan King, Jim Scott, Kathy Gunst)
Makes 6-12 Squares
For the Crust
1 1/2 cups all-purpose flour
1/2 cup confectioners' sugar, sifted
1/4 teaspoon salt
3/4 cup (1 1/2 sticks) unsalted butter, cold, cut into 8 pieces
For the Filling
1/2 cup raspberry jam, with or without seeds
4 cups fresh or frozen raspberries
2 to 4 tablespoons all-purpose flour
For the Topping
1/2 cup (1 stick) unsalted butter, cold, cut into 1/2 inch cubes
1 cup all-purpose flour
1 cup packed light brown sugar
Place a rack in the middle of the oven and preheat the oven to 375 degrees.
To make the crust: Combine the flour, confectioners' sugar, salt and butter in a food processor. (I used a stand mixer with the paddle attachment which works just as well.) Pulse about 30 times, or work the butter in a pastry blender or your fingers, until the butter is the size of small peas. Sprinkle the mixture into an ungreased 9x13-inch baking pan. Using your hands, press the mixture into the bottom of the pan about 1/2 inch up the sides of the pan. Bake the crust for 10 to 12 minutes, or until firm and barely beginning to brown.
Meanwhile, make the filling: Melt the jam in a small saucepan over medium heat, just until thinned out and liquefied.
In a medium bowl, gently mix the raspberries with enough flour until fully coated, and set aside.
Make the topping: Combine the butter, flour, and brown sugar in a food processor (again, I used the mixer with the paddle attachment) or medium bowl and pulse and work the butter in with a pastry blender or your fingers until the mixture begins to come together and starts to move away from the sides of the bowl; set aside.
When the crust is done, remove it from the oven. Use the back of a kitchen spoon or a pastry brush to spread the jam evenly over the hot crust. Sprinkle the berries and any flour remaining in the bowl evenly over the jam. Using your hands, sprinkle the topping evenly over the berries, breaking up any clumps larger than a nickel. Bake for 25-30 minutes, or until the berries are bubbling and the topping is brown. Remove from the oven and cool completely before cutting into squares.