Begin by making the preserves. In a medium-sized saucepan over medium to high heat, combine the blueberries, granulated sugar, cornstarch, and fresh squeezed lemon juice.
Using a potato masher, mash the berries until they are slightly chunky but more on the puréed side. You can also leave larger chunks of blueberries if you prefer that.
Stir the preserves with a spoon for about 3 to 5 minutes or until they begin to thicken. Remove from heat and allow to cool to room temperature either in the refrigerator or on the counter on a hot pad.
For the crumble, combine all ingredients except the butter in a medium to large mixing bowl, and whisk for about 20 seconds. Chop butter into tablespoon size pieces, and add to the mixture. Using a pastry cutter or a spoon by hand, combine until a crumbly mixture forms, about a minute or so.
Remove 2 cups of the mixture and place in a different bowl. Take the remainder of the mixture and pour into the prepared pan, pressing firmly. Using the back end of a larger measuring cup, press firmly down until mixture is evenly set in the pan. Next, pour the preserves over the crust portion of the mixture, and spread evenly across crust.
Lastly, crumble the remaining 2 cups of the mixture over the preserves, ensuring that you do not press down as you want these to become golden and stay more crumbly.
Bake at 350° for 40-50 minutes, or until slightly golden on top. Remove an allow to cool for 15 to 20 minutes. You can serve these bars warm and they will be incredibly soft and crumbly, or you can allow them to go completely and they will be more stable. They will take about two hours to cool completely. Serve as is or top with vanilla bean ice cream. Store any leftovers in an airtight container at room temperature for up to four days. Enjoy!