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 1-2 tbsp water to form a soft dough. Knead well then wrap & chill for at least 30 mins.
Pre-heat the oven to 200°C/gas 6/fan 180°C. Knead the pastry again & 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 & brush the base & 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 goats cheese.
Season and if you like, add a pinch of nutmeg. Put the flan tin on a baking sheet and bake pastry blind for 5 mins. Add the filling and bake for 15 mins. Reduce heat to 180°C/ gas 4/ fan 160°C and cook for 25-30 mins or until lightly set.
All ingredients used in this recipe are available to purchase from the Fodder farm shop