Sift and combine the flour, salt, and sugar into a small bowl. Gradually add the melted butter while stirring with a fork, until the mixture starts clumping together. Add the rolled oats, stir to combine, and set aside until needed.

Peel and dice the apples. Place the apples in a large bowl and add the lemon zest, juice and blackberries.

In a small bowl, combine the sugar, cornstarch, cinnamon and cardamom until every granule is evenly coated.

Add the sugar mixture to the fruit and gently stir everything with a spoon until completely incorporated.

Preheat the oven to 190°C (375°F) or 175°C (350°F) if using a fan-assisted oven.

Add the prepared fruit filling to a baking dish lightly greased with butter. Pour over all the juices from the bowl.

Sprinkle the top with prepared streusel.

Bake on a middle oven rack for about 40-45 minutes, or until the top is golden and the bubbles from the fruit are peeking through the crumble.

When done, remove the crumble from the oven and let it cool slightly before serving with some ice cream or freshly whipped cream.