For the filling: In a small saucepan, mix together sugar and cornstarch. Add blueberries, lemon juice, lemon zest, and water. Stir to combine. Bring to a simmer over medium heat, stirring constantly. Reduce heat to medium low and continue to cook until thickened and berries are mostly broken down (5 minutes).
In a large mixing bowl, combine oats, flour, brown sugar, cinnamon, and salt until well combined. Add butter and use a pastry cutter or two forks or knives to cut in the butter until the mixture is crumbly and there are no large chunks of butter remaining.
Spray a 9×13-inch baking pan or dish with nonstick cooking spray.
Set aside 2 cups of crumb mixture. Put the remaining crumb mixture into the prepared pan and press down firmly in an even layer, using your fingers or the back of a large spoon.
Bake for 10 minutes. Remove pan from oven and carefully spread fruit filling evenly over crust. Sprinkle with reserved crumb mixture and gently pat down with a large spoon. Put the bars back in the oven and bake for 40 to 50 minutes or until golden brown.
Cool completely before cutting into bars.
Video
Notes
Storage: Store in an airtight container (away from heat and sunlight) for 4 to 5 days. If you want to keep them longer, refrigerate the bars in an airtight container for up to 7 days. To freeze them, individually wrap each bar in plastic wrap and then in foil once. Transfer the wrapped bars into a freezer-friendly bag or container for up to 3 months. Thaw them overnight in the fridge or set them on the counter until they come to room temperature.
If you'd like, you can use different berries, or a combination of berries. Combining your favorite berries like strawberries, blackberries, raspberries, pitted cherries, and blueberries makes a uniquely different filling.