Make It Now:

1. Prep: Preheat oven to 375° F. Line a 9×13 inch baking pan with foil or parchment paper, leaving an overhang on opposite sides to use as “handles” to lift the bars out afterward. Spray the foil/parchment with nonstick cooking spray.
2. Make the Shortbread Crust: Add the flour, sugar, and salt to the bowl of a stand mixer. Mix briefly on low speed to combine.
3. Cut 16 tablespoons (2 sticks) of the butter into small, 1/2-inch pieces. With the mixer on low, add of the butter, one piece at a time. When you’ve finished adding the butter, continue mixing on low speed until the butter has been broken down into small pieces and the mixture resembles wet sand (shouldn’t take much longer than a minute or two).
4. Important! Transfer 1 1/4 cups of this crust mixture to a small mixing bowl and set aside.
5. Dump the remaining crust mixture into the prepared pan and press into an even layer over the bottom of the pan. 
6. Bake for about 15 minutes, or until the edges of the crust turn golden brown and it is set.
7. Make the Streusel Topping: Add the oats and brown sugar to the bowl with the reserved crust mixture. Stir to combine. Add the remaining two tablespoons of butter to the bowl and use your fingers (or a pastry cutter) to work the butter into the dry ingredients until combined.
8. Make the Filling: Stir together the blueberry preserves (or jam), fresh blueberries and lemon juice in a small mixing bowl. Mash the blueberries with a fork or potato masher. You want to break them down some, but still leave some chunks resembling berries.
9. Finish the Bars: When you remove the crust from the oven, add the filling directly on top of the hot crust. Spread in an even layer. Sprinkle the streusel topping over the filling evenly. Don’t press the streusel into the filling!
10. Bake for 20-25 minutes, or until the filling is bubbling and the streusel has browned.
11. Remove the pan and let the bars cool to room temperature, about 1-2 hours. Use the foil handles to lift the bars from the pan and cut into squares for serving. Store in an airtight container at room temperature.

Freeze For Later: Bake bars according to recipe directions. Let cool completely. Once cool, cut bars apart. Store in freezer bag or wrap tightly with plastic wrap and freeze.

Prepare From Frozen: Remove desired number of bars. Thaw overnight in the refrigerator or warm in 10 second increments in the microwave.