Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9-inch pie dish or similar oven-safe baking dish.
Step 2: In a large bowl, combine the blackcurrants (if using frozen, no need to thaw), granulated sugar, cornstarch, and optional lemon juice. Toss gently until the berries are well coated. Pour the fruit mixture into the prepared baking dish.
Step 3: In a separate large bowl, whisk together the all-purpose flour, rolled oats, light brown sugar, and salt.
Step 4: Add the cold, cubed butter to the dry ingredients. Using your fingertips, a pastry blender, or two knives, cut or rub the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining. Avoid overworking the mixture.
Step 5: Evenly sprinkle the crumble topping over the blackcurrant filling in the baking dish. Do not press down on the topping; keep it light and airy.
Step 6: Place the baking dish on a baking sheet (to catch any potential drips) and bake for 35-45 minutes, or until the crumble topping is golden brown and the blackcurrant filling is bubbling around the edges. If the topping browns too quickly, you can loosely cover it with aluminum foil for the last 10-15 minutes.
Step 7: Remove from the oven and let cool for at least 15-20 minutes before serving. This allows the fruit filling to set and prevents it from being too runny. Serve warm with your favorite accompaniments.