Put the flour and a pinch of salt in a large bowl. Rub in the butter until the mixture looks like breadcrumbs. Stir in the egg and about 1-2 tablespoons water to form a soft dough. Knead well then wrap and chill for at least 30 minutes.
Preheat the oven to 400 degrees F. Knead the pastry again and roll it out to line a 20cm loose-bottomed flan tin (the pastry won't shrink, so no need to chill). Patch holes with spare pastry and brush the base and sides with egg.
For the filling, fry the onion in the oil until just soft, add the watercress and cook until just wilted. Beat the eggs and milk and stir in the watercress mixture and goat's cheese.
Season and if you like, add a pinch of nutmeg. Put the flan tin on a baking sheet and bake the pastry blind for 5 minutes. Add the filling and bake for 15 minutes. Reduce heat to 350 degrees F and cook for 25-30 minutes or until lightly set.