Set a pot with 500 ml water on the stove and bring it to a boil.
Preheat the oven to 200°C, line a baking tin with baking paper and press a puff pastry form into it.
Peel the sweet potato and cut it into 2-mm slices. Cut the cheese in slices that are the same thickness as the sweet potato's.
When the water boils, add the salt and the sliced sweet potato. Boil the potato for 5 minutes, then drain and set aside.
Crack the egg in a bowl and add the mustard. Whisk well, then add the milk and give another whisk to combine. Pour this mixture over the bottom of your puff pastry crust, distributing it all over the surface.
Allow the sweet potato to cool for a couple of minutes, then lay sweet potato and cheese slices, alternating, over the egg mixture in the crust.
Bake in the preheated oven at 200°C for 20-25 minutes.