Preheat the oven to 375 degrees F. Line a 9-by-13-inch baking pan with foil so that the foil over hangs the sides by a few inches, and then butter the foil.

In a small bowl, combine the fresh or frozen blueberries, preserves, and lemon juice. Use a fork to mash the berries so that a bit of the fruit remains. Stir to combine, and set aside.

Add 16 tablespoons of butter (leaving 2 tablespoons for later), flour, sugar, and salt to the bowl of a stand mixer or in the food processor. Process until the mixture becomes a uniform texture and color, about 1 minute.

Reserve 1 1/4 cups of the butter mixture for the topping. Press the remaining butter mixture into the foil-lined pan.

Press the mixture together in the bottom of the pan using the bottom of a cup. Bake the crust until the edges start to brown, around 15 minutes.

Spread the blueberry mixture evenly over the hot crust.

Add the oats, brown sugar, and remaining 2 tablespoons of softened butter to the reserved flour mixture, and stir to combine.

Pinch the mixture between your fingers to make small clumps of streusel, and drop them evenly over the fruit. Once you've made lots of streusel lumps, sprinkle any remaining topping over the fruit.

Bake the bars until the blueberry filling begins to bubble and the topping is a deep golden color, around 24 minutes.

Remove from the oven, and let the bars cool completely in the pan.

Remove the bars from the pan by pulling on the foil, cut into squares, and serve.

Store any extras in an airtight container.