Grease and line a 20cm medium height loose bottomed/spring form tin and set aside.
Pierce the sweet potatoes, place on a heat proof plate and microwave for around 6 minutes, turning half way through. Gently squeeze to see if they are cooked, they should be nice and soft, if not cook for a further minute or so until they are. Remove from the microwave and allow to cool. Once cooled, cut in half and scoop out the flesh and place into a mixing bowl.
Add the Parmesan, breadcrumbs, garlic powder, egg yolk and seasoning and mix well to combine. Place into the baking tin and spread evenly around the bottom and sides to create an even thickness shell. Place into the freezer for 2-3 hours.
3 Cheese Filling
In the meantime make your filling. Reserve a small handful of the 3 cheeses for later. In a large bowl, mix the remaining ingredients together and place in the fridge ready to use later.
Preheat the oven to 180*C. Remove the sweet potato crust from the freezer, place on an oven tray and bake in the oven for 20 minutes.
Remove from the oven and egg wash the inside carefully, place back into the oven and bake for a further 10 minutes. Once cooked, allow to cool slightly before popping into the fridge for 20 minutes.
After it has had time to set and cool, pour the cheese filling into the case, top with reserved cheese and bake on 160*c for 40 minutes. Half way through cover with foil so the top doesn't brown too much, top with charred spring onions and continue to cook until the filling has set. Allow to cool fully before carefully removing from the tin and serving with a simple green salad.