Preheat the oven to 350°F (180°C). Peel, core, and slice the apples, then place them in a 10 ½ inch (26cm) pie dish. Toss the sliced apples with lemon juice, demerara sugar, ground cinnamon, and plain flour until the apples are evenly coated. This helps infuse the apples with flavor and keeps the filling from becoming too watery.

Gently fold the blackberries into the apple mixture in the pie dish so they are distributed throughout. Dot the mixture with the cold cubed butter. Cover the pie dish with foil or a large oven-safe lid and bake in the preheated oven for 20 minutes to soften the fruit and meld the flavors together.

While the fruit is baking, prepare the crumble topping. If using Biscoff cookies, crush them gently in a food processor or in a bag with a rolling pin; set aside. In a mixing bowl, combine the self-rising flour, golden caster sugar, and ground cinnamon. Add the chilled unsalted butter and rub it into the dry ingredients with your fingertips until the mixture forms a crumbly texture that holds together in small lumps. If the texture feels too dry, add a touch more butter; if too greasy, add a sprinkling more flour. I like to leave a few bigger clumps in the topping for a rustic, crunchy texture.

Remove the partially baked fruit filling from the oven using oven mitts. Evenly spread the crumble topping over the warm fruit, pinching sections of the topping into small clumps for texture. Sprinkle the crushed Biscoff cookie crumbs over the top. Return the dish to the oven and bake for another 30 minutes, or until the crumble is golden and the fruit filling bubbles up at the edges.

Allow the crumble to cool for several minutes after baking; this helps the filling set and makes it easier to serve. Spoon the warm fruit crumble into bowls and serve as is, or with a generous scoop of vanilla ice cream, custard, or cream. For a touch of extra comfort, I like to serve mine with warm custard.