Preheat the oven to 350˚F (180˚C). Grease a 9-inch (23 cm) loose bottomed pie or tart pan.
For the walnut pie crust, in a food processor or blender, pulse the walnuts, flour, sugar, and salt until fine. Add the butter and pulse again until the mixture forms a crumbly dough. Press this into the base and side of the prepared tin, then place this on a baking sheet and bake for 15–20 minutes, until lightly browned. Remove from the oven and let cool.
Increase the oven temperature to 375˚F (190˚C).
To make the filling, place the sweet potato in a saucepan and add ½ cup (120 ml) of water. Cover and cook over a medium heat for 15–20 minutes, until the sweet potato is very soft. Drain.
Return the sweet potato to the saucepan, along with the brown sugar, coriander, nutmeg, cinnamon, salt, butter, and half the evaporated milk. Stir, and cook for about 5 minutes. Using a food processor or blender, blend the sweet potato mixture until very smooth. Allow to cool.
Whisk the remaining evaporated milk together with the eggs and vanilla extract. Pour this egg mixture into the cooled sweet potato mixture and stir to combine. Pour the sweet potato filling into the prepared pie crust, place on a baking sheet and bake for 10 minutes. Reduce the heat to 325˚F (165˚C) and cook for a further 35–40 minutes, until the filling is cooked through and the center wobbles slightly when jiggled. Remove from the oven and leave to cool—it can be eaten warm, or at room temperature.
Serve each slice with a little cream or vanilla ice cream.