6 cups regular rolled oats
2-1/4 cups packed light brown sugar
2-1/4 cups all-purpose flour
2 cups unsalted butter, melted, plus more for pan
1 teaspoon kosher salt
1 teaspoon baking soda
9 cups fresh blackberries or other fruit (strawberries, peaches, apples)
1 cup granulated sugar
1/2 cup fresh lemon juice (from 3 lemons)
1/4 cup cornstarch
1. Prepare the crust: Preheat oven to 350 degrees. Stir together oats, brown sugar, flour, butter, salt and baking soda. Remove 3 cups of the oat mixture and set aside. Press remaining oat mixture in an even layer in an aluminum foil-lined and buttered 12- x 17-inch rimmed baking sheet. Bake in preheated oven until golden brown, about 20 minutes. Cool completely, about 30 minutes.
2. Prepare the filling: Stir together blackberries, sugar, lemon juice and cornstarch in a large saucepan. Cook over medium heat, stirring occasionally, until syrup thickens and is bubbly, about 8 minutes. Pour over prepared crust and crumble reserved 3 cups oat mixture evenly over filling. Bake at 350 degres until topping is golden brown, 30 to 35 minutes. Cool completely, about 2 hours. Cut into 24 bars.