Classic apple crumble

Name: Classic apple crumble
Cuisine: European, British
Ingredients: - 750 g/ 1 lb 10 oz Bramley (cooking) apples, peeled, cored and diced; - 250 g/9 oz eating apples, such as Granny Smith, Cox's or Braeburn, peeled, cored and diced; - 75 g/ 2¾ oz (⅓ cup) light brown sugar or caster (superfine) sugar; - lemon juice, to taste; - Custard (page 33), to serve; - 200 g/7 oz (1⅔ cups) plain (all-purpose) flour; - 150 g/5 oz (1¼ sticks) salted butter, diced; - 100 g/3½ oz (1 cup) rolled oats; - 175 g/ 6 oz ¾ cup) demerara (turbinado); or granulated sugar; - 50 g/ 2 oz (½ cup) nuts of choice, chopped (optional)