Preheat your oven to 375°F (190°C). In a large mixing bowl, gently toss the blackberries with sugar and lemon juice, coating them evenly. Transfer the berry mixture to a baking dish, spreading it out in an even layer.
In a separate bowl, combine oats, flour, and brown sugar. Mix briefly to distribute evenly. Add the cold, cubed butter and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces.
Sprinkle the crumble topping evenly over the berries in the baking dish, pressing it down lightly with your hands to help it adhere and create a uniform layer.
Place the dish in the oven and bake for about 35 minutes, or until the topping is golden brown and crisp, and the filling is bubbling around the edges. The aroma of toasted oats and baked fruit will fill your kitchen.
Remove the blackberry crisp from the oven and let it cool slightly for about 10 minutes. The filling will thicken as it cools, and the topping will stay crisp.
Serve warm, scooping out generous portions that reveal the juicy berries beneath the crunchy topping. Enjoy on its own or with a scoop of vanilla ice cream for an extra treat.
