Measure the blackberries and place them in a mixing bowl. Use a fork to mash some of the berries, just enough to release their juice but still leave some whole berries for texture. Stir in the sugar and lemon juice, then set aside to macerate while preparing the topping.
In another bowl, combine the oats, flour, and a pinch of cinnamon or nutmeg if using. Add the cold butter and use your fingertips or a pastry cutter to rub the butter into the dry ingredients until the mixture resembles coarse crumbs. This creates a buttery, crumbly topping with small clumps throughout.
Pour the prepared blackberry mixture into a baking dish, spreading it out evenly. Then, sprinkle the crumble topping evenly over the berries, covering the surface completely. Gently press the topping down to ensure good contact with the fruit beneath.
Place the baking dish in a preheated oven at 375°F (190°C). Bake for about 40 minutes until the topping is golden brown and the filling is bubbling around the edges. The aroma of baked berries and butter will fill your kitchen as it bakes.
Remove the crisp from the oven and let it cool slightly for 10 minutes. The topping will be crisp and golden, while the filling remains juicy and fragrant. Serve warm, ideally with a scoop of vanilla ice cream or a dollop of whipped cream for extra indulgence.
