Preheat the oven to 350 degrees F and line a 8” x 8” cake pan with parchment paper.

Stir together all of the ingredients for the crust and topping in a mixing bowl. Keep stirring until a thick, moist, crumbly dough forms. Measure out ½ cup of the crust mixture and place it in a bowl to use as the topping. Add ⅓ cup of chopped walnuts (or pecans) to the bowl (if desired).

Press the rest of the crust mixture into the parchment-lined cake pan until you have an even layer of crust. Bake on the center rack of the preheated oven for 10 to 12 minutes, or until crust is golden brown and appears as though it has set up. Remove from the oven and allow crust to cool.

While the crust is baking and cooling, prepare the blueberry filling. Add all ingredients for the filling to a small saucepan, cover, and heat over medium. Bring mixture to a full boil. Stir, mash the blueberries, and cook until mixture is thick and gooey, about 5 to 8 minutes. Remove from heat and set aside or refrigerate until ready to use.

Once the crust has cooled, spread the blueberry mixture over the crust, followed by the topping. Note: I like to use my hands to squeeze the topping mixture together into larger clumps, but you can also just sprinkle it right on.

Bake 20 to 25 minutes, until crumb bars are golden-brown and bubbly. Remove from the oven and allow crumb bars to cool at least 30 minutes before slicing and serving.