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
1 cup granulated sugar
1/2 cup fresh lemon juice (from 3 lemons)
1/4 cup cornstarch
How to Make It
Prepare the Crust: Preheat oven to 350°F. 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.
Prepare the Filling: Stir together blackberries, sugar, lemon juice, and cornstarch in a large saucepan. Cook over medium, 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°F until topping is golden brown, 30 to 35 minutes. Cool completely, about 2 hours. Cut into 24 pieces.