Mash the cooked sweet potato with a fork in a bowl. Add the remaining ingredients for the base and mix well with a spoon.
Wearing disposable gloves, spread the mixture on the bottom and sides of a 24 cm (9 ½-inch) round tart tin with a detachable base. Transfer to the freezer for 2 hours.
Preheat the oven to 180°C/356° F, fan.
Bake the base for 20 minutes. When it is ready, brush it with the diluted yolk and bake for another 10 minutes. Once ready, set aside.
For the filling:
Put all the filling ingredients in a bowl and mix well with a hand whisk until the mixture is homogeneous. Pour the filling onto the baked base and sprinkle with cheddar and parmesan.
Lower the oven temperature to 160° C/320° F, fan, and bake for 35-40 minutes.
When ready, allow to cool, remove from the tray, and serve.
Notes
Chef’s tip: Feel free to add any grated cheese of your choice.