Preheat the Oven: Preheat your oven to 350°F (175°C). This ensures that the oven is at the correct temperature when you’re ready to bake the crumble.

Make the Topping: In a medium bowl, combine the oats, flour, brown sugar, and salt. Stir these dry ingredients together to combine them evenly.

Add the butter (cut into tablespoon-sized pieces) to the dry ingredients. Use a pastry blender or two forks to cut the butter into the dry ingredients until the mixture becomes crumbly and resembles small pea-sized clumps. This step creates the crumbly topping for the crumble.

Once the topping mixture is ready, set it aside.

Prepare the Filling: In another bowl, combine the blackberries, sugar, flour, and lemon juice. Stir gently to coat the berries evenly with the ingredients. The flour helps thicken the fruit filling as it bakes, while the lemon juice brightens the flavor of the blackberries.

Assemble the Crisp: Lightly grease a 9×13-inch baking dish. This will help ensure the crumble doesn't stick to the dish while baking.

Spread the blackberry mixture evenly in the bottom of the prepared baking dish.

Evenly sprinkle the oat topping over the blackberries. Make sure the topping covers the entire fruit mixture for an even bake.

Bake the Crumble: Place the baking dish in the preheated oven and bake for 45 minutes. The crumble is done when the fruit filling is bubbling around the edges and the topping is golden brown.

If the topping isn't golden enough after 45 minutes, you may need to bake for an additional 5-10 minutes. Keep an eye on the crumble to prevent overbaking.

Cool and Serve: Once baked, remove the crumble from the oven and let it cool for about 10-15 minutes. This cooling time allows the fruit layer to thicken slightly, making it easier to serve.

Serve the blackberry crumble warm or at room temperature. For an extra indulgence, serve it with a scoop of ice cream, a dollop of lemon frozen yogurt, or a spoonful of sweetened whipped cream.