To make the crust bake the sweet potatoes until tender.
Preheat oven 200c/180 fan assisted.
When cool enough to handle cut the sweet potatoes in half and scoop out the flesh into a bowl (approximately 300g yield) and mash well.
Stir in the flour with a pinch of salt and 3 tbs of olive oil. Bring the mixture together and it should resemble a dough, if too soft add a little more flour.
There is no need to roll out just press the mixture into the base and sides of a greased pie dish or quiche dish (23cm /9inch). Brush the surface with the remaining oil and bake for 20mins until the edge is crisp.
Remove from oven.
Reduce oven to 180/160 fan assisted.
Turn the salmon onto a plate and break down into small chunks removing all bones and skin place evenly over the crust and scatter over the watercress.
Beat the eggs and the milk/cream together add more milk depending on how liquid the mixture becomes. Add seasoning pour over the pastry case and bake for 30-40mins until the filling is set.
Serve warm or cold with salad.