Apple Crumble

For the filling

• 4 green sour apples (Granny Smith), about 700g
• 4 red apples (e.g. Starking or Red Delicious), about 700g
• 2 tablespoons (20ml) lemon juice
• ¼ pack (65g) unsalted butter
• ⅔ cup (90g) light brown soft sugar
• 1 teaspoon (4g) cinnamon
• 1 tablespoon (10ml) vanilla extract

For the crumble

• 1 ½ cups (200g) all-purpose flour or “00” flour
• ⅔ cup (90g) light brown soft sugar
• 3 tablespoons (45g) granulated sugar
• 1 pinch salt
• 1 teaspoon (4g) cinnamon
• ½ pack (125g) cold unsalted butter, cubed

For the topping

• ½ cup (65g) roughly chopped walnuts

For the butter caramel sauce

• 1 cup (230g) sugar
• ⅓ pack (85g) unsalted butter
• ½ cup (125ml) heavy cream
• ⅓ teaspoon salt
• ½ teaspoon ground cinnamon

For serving

• Vanilla or mastiha ice cream