Preheat Oven & Prepare Dish: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Lightly grease a 20cm x 20cm (or similar sized) baking dish. The oven will start to warm, filling your kitchen with a gentle heat.

Make the Crumble Topping: In a large bowl, combine the plain flour, demerara sugar, ground cinnamon, ground nutmeg, and a pinch of salt. Add the chilled, cubed butter. Using your fingertips, rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs, with some larger pea-sized clumps remaining. The mixture will feel sandy and slightly moist, and you should smell the fragrant spices.

Prepare the Blueberry Filling: In another bowl, gently combine the fresh blueberries, granulated sugar, cornflour, lemon zest, lemon juice, and optional vanilla extract. Toss them carefully until the blueberries are evenly coated and glistening with the sugar mixture.

Assemble the Crumble: Pour the blueberry mixture into the prepared baking dish, spreading it out into an even layer. The blueberries will look plump and vibrant purple in the dish. Evenly scatter the crumble topping over the blueberries, ensuring the fruit is mostly covered.

Bake the Crumble: Place the dish in the preheated oven and bake for 35-40 minutes. Keep an eye on it! The crumble is ready when the topping is golden brown and crisp, and the blueberry filling is bubbling thickly around the edges, letting off a sweet, fruity aroma that will fill your entire home.

Cool and Serve: Once baked, carefully remove the crumble from the oven. Let it rest for at least 10-15 minutes before serving. This allows the fruit filling to set slightly, preventing it from being too runny. You’ll notice the bubbling subsides as it cools, and the aroma becomes even more inviting.