• Preheat the oven to 350°F, and lightly grease a 9x13-inch baking dish.

To Make the Crumble Topping:

• In a medium bowl, combine the oats, flour, brown sugar, granulated sugar, cinnamon, and salt.
  1 cup old-fashioned rolled oats, ½ cup all-purpose flour, ½ cup brown sugar, ¼ cup granulated sugar, ½ teaspoon ground cinnamon, ¼ teaspoon salt
• Add the cold, cubed butter and use clean hands or a pastry cutter to work it in until the mixture is crumbly and holds together when pressed.
  6 tablespoons unsalted butter
• Place the topping in the refrigerator to chill while you prepare the filling.

To Make the Filling:

• In a large bowl, toss the frozen blackberries with the sugar, cornstarch, lemon juice, vanilla, and cinnamon until evenly coated.
  32 ounces frozen blackberries, 1 cup granulated sugar, 4 tablespoons cornstarch, 1 tablespoon lemon juice, 1 teaspoon vanilla extract, ½ teaspoon ground cinnamon
• Spread the mixture into the prepared baking dish.
• Remove the chilled crumble topping from the fridge and sprinkle it evenly over the blackberry filling.
• Bake for 45 to 55 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
• Let it cool for 15 to 20 minutes before serving. Serve warm, optionally with vanilla ice cream.