Preheat your oven to 180°C (160°C fan) or Gas Mark 4.
Prepare the blackberries, Rinse if fresh, or defrost if frozen. Toss them with the 50g sugar and optional lemon juice. Spread evenly into a baking dish.
500 g Blackberries, 50 g Caster sugar, 1 tbsp Lemon juice
In a bowl, rub the cold butter into the flour using your fingertips until the mixture resembles coarse breadcrumbs.
150 g Plain flour, 100 g Unsalted butter, 75 g Demerara or caster sugar
Stir in the remaining sugar and oats (if using).
50 g Rolled oats
Spoon the crumble topping over the blackberries evenly. Do not press it down—this helps it bake with a lovely texture.
Place the dish in the oven for about 35–40 minutes, until the topping is golden and the fruit juices are bubbling at the edges.
¼ tsp Ground cinnamon or nutmeg
Cool slightly before serving.