Preheat the oven to 375 degrees. Line a 9x13 inch baking pan with a foil sling and then grease the foil (have the foil over hang the edges so that you can remove the bars to cut and serve).

Whisk the flour, granulated sugar, and salt together in a large bowl. Add 16 of the butter pieces (1 cup, save out 2 tablespoons). Use an electric mixer (or a stand mixer) and beat on low until the mixture starts to resemble wet sand, about 2 minutes. Remove 1 1/4 cups of the mixture and place it in a seperate bowl (reserve for topping).

Place the rest of the butter and flour mixture in the baking pan and press it into an even layer with the bottom of a measuring cup. Bake the crust until it starts to smell good and the edges begin to brown, 14 to 18 minutes.

While the crust is cooking mix the reserved flour mixture, oats, nuts, and brown sugar together. Add the remaining 2 tablespoons of butter and pinch the mixture between your fingers to make large pea-sized streusel.

In a small bowl mash the jam, berries, and lemon juice together, leaving a few chunks of raspberry.

Spread the berry mixture evenly over the hot crust, sprinkle on the topping. Bake until filling is bubbling and the topping is a deep golden brown, 22 to 25 minutes.

Let the bars cool completely in the pan on a wire rack for 2 hours. Remove from the pan using the foil and then cut into squares and serve.