Step 1: Preheat your oven to 375°F (190°C) and lightly grease a 9×9 inch baking dish or a 2-quart casserole dish.
Step 2: In a large bowl, gently toss the fresh blackberries with the granulated sugar, lemon juice, and cornstarch until the berries are evenly coated.
Step 3: Pour the berry mixture into the prepared baking dish and spread it out into an even layer.
Step 4: In another bowl, combine the flour, rolled oats, brown sugar, cinnamon, and salt for the topping of the blackberry crisp recipe.
Step 5: Using a pastry cutter or two forks, cut the cold, cubed butter into the dry mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 6: Sprinkle the oat topping evenly over the blackberries in the baking dish.
Step 7: Bake for 40-45 minutes, or until the topping is golden brown and the berry juices are bubbling thickly around the edges and in the center.
Step 8: Remove from the oven and allow the blackberry crisp recipe to cool for at least 15 minutes before serving to let the filling set.