Preheat the oven to 200°C/400°F/Gas 6.
Place the pie crust into a baking/pie dish and set it aside.
To make the filling, core, peel, and slice the apples into ¼"/0.5cm slices and place them into a mixing bowl.
Add the lemon juice and water to cover all of the apple slices. Leave to sit for 5 minutes. Drain and rinse the apple slices and return to the bowl
Add flour, cloves, nutmeg, cinnamon, sugar, and vanilla extract to the bowl of apples, and use a rubber spatula to combine the ingredients together. Spoon the filling into the pie crust and set it aside.
To make the crumble topping, place the brown sugar, cinnamon, flour, and walnuts into a mixing bowl and use a rubber spatula to mix the ingredients together. Then mix in the butter to form the crumble as shown.
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Sprinkle the topping over the top of the pie and place it in the preheated oven. Bake for 20 minutes.
Do not remove the pie from the oven but reduce the oven temperature to 190°C/375°F/Gas 5 for another 30-35 minutes.
Remove the pie from the oven and leave it to cool for 3 hours.
To make the chocolate ganache, place the chocolate chips into a medium bowl.
Pour the heavy whipping cream into a small pan and heat until just simmering.
Pour the warmed cream over the chocolate chips and whisk until smooth. Leave to sit for 5 minutes.
Spoon the ganache into a disposable piping bag and cut the tip off the bag.
Drizzle the chocolate ganache over the top of the cooked and cooled pie and sprinkle over the ¼ cup of chocolate chips if using.
Leave the chocolate ganache to set before serving. Enjoy!