Spiced Apple and Walnut Crumble Cake

Ingredients

• 2 Granny Smith apples, peeled, cored and sliced
• 1 tbsp caster sugar
• Juice of ½ a lemon
• 2 tbsp unsalted butter
• ¼ cup plain flour
• ¼ cup brown sugar
• ¼ tsp ground cinnamon
• ¼ cup walnuts, chopped
• ¼ cup rolled oats
• 1 ½ cups plain flour
• ½ tsp baking powder
• ½ tsp bicarbonate of soda
• ½ tsp salt
• 1 tsp ground cinnamon
• ½ tsp ground ginger
• 1/8 tsp ground nutmeg
• 1 cup brown sugar
• ½ cup vegetable oil
• 2 tsp vanilla extract
• 2 eggs
• ½ cup sour cream