Preheat oven to 325°F.

In a small saucepan, mix together sugar and cornstarch. Add remaining filling ingredients (1 cup blueberries, 1 cup blackberries, 2 tablespoons lemon juice, 1 teaspoon lemon zest and 3 tablespoons water). Stir to combine. Bring to a simmer over medium heat and stir frequently. Reduce heat to medium low and continue to cook until thickened and berries are mostly broken down (10-15 minutes).

Meanwhile, prepare dough for the bars. In a large mixing bowl, combine oats, flour, brown sugar, cardamom, 1 teaspoon lemon zest, and salt until well combined. Sprinkle mixture with 2 tablespoons fresh lemon juice. Add butter and use a pastry cutter or two forks or knives to cut in the butter pieces until the mixture is crumbly and there are no large chunks of butter remaining.

Spray a 9×13-inch pan with nonstick cooking spray.

Set aside 2 cups of crumb mixture. Pour the rest into the prepared pan and press down with fingers to form a uniform crust. Bake for 10 minutes or until set. Spread fruit filling evenly over crust. Sprinkle with reserved oat mixture and press down gently (be careful for the hot pan!).

Bake for an additional 35-40 minutes or until golden brown.

Cool completely before cutting into bars.