Place walnuts into your food processor for 30 seconds or so, or until they're finely ground. Add flour and sugar and pulse; then, add the butter and pulse again. Last, add in the ice water and pulse until dough comes together and is no longer crumbly.
Remove dough from the processor and form it into a round disc. Wrap it in plastic wrap and stick it in the refrigerator for at least 30 minutes or longer if preferred.
Once the 30 minutes have passed, remove dough and preheat oven to 350°.
Roll dough out into a circle and carefully transfer it to your pie or tart pan, using your hands to press it gently into the pan and form the edges of the crust.
For the filling: combine all ingredients in your blender and pulse until smooth. Use a rubber spatula to scoop mixture out into the tart/pie pan, smoothing it out.
Bake pie for 45 minutes or so. If you're using a tart pan, wait 5 minutes and then remove pie from the pan. If you're using a pie pan, simply wait for pie to cool--I recommend chilling in the refrigerator--then slice and serve with whipped cream (optional).
For the whipped cream: the coconut milk should be mostly cream at this point, but scoop out the cream and drain out any liquid left in the can (you just want the cream part); then, add sugar and cinnamon and beat with an electric mixer until smooth.
Top pie with whipped cream and enjoy!
*If mixture isn't coming together, add extra water in 1 tbsp at a time