Heat the oil in a pan and flour. Stir well until it is smooth. Add the milk and whisk until it is completely smooth and the sauce thickens. Season with nutmeg, salt and pepper.
Preheat the oven to 200 degrees Celsius.
Oil an oven proof dish and arrange the artichoke, onion and sweet potato in layers. Pour the sauce over the vegetables, cover with foil and bake for 1 hour.
Increase the heat to 220C. Remove the foil and sprinkle grated parmesan over the top and bake for a further 5 - 10 minutes until top is nicely browned and crusty.
Sprinkle the parsley on top and serve.
Note : Jerusalem artichokes will discolour if they are cut and exposed to oxygen for too long. To avoid this you can sit them in water with lemon juice.