1. Bring a small pan of water to the boil and add the squash. Cook for 15 minutes or until tender then drain. Press through a sieve into a large bowl to make a purée, then set aside to cool.
2. Preheat the oven to 160ºC, gas mark 3 and place the pastry case onto a baking sheet. When the squash is cool, add the maple syrup, egg, cream and nutmeg, and mix well. Pour the mixture into the pastry case and bake in the oven for 40 minutes or until just set.
3. Leave the pie to cool completely at room temperature. Meanwhile, place the walnuts, half the sugar and cinnamon into a mini processor and pulse until the nuts are finely chopped. Stir in the remaining sugar. Sprinkle over the tart before cutting into slices and serving with cream.