Goat's cheese & watercress quiche

INGREDIENTS

• 225g gluten-free flour , plus extra for rolling
• 100g butter
• 1 large egg , beaten, plus extra beaten egg for brushing
• 1 medium onion , finely chopped
• 1 tbsp oil
• 100g watercress , roughly chopped
• 2 large eggs
• 150ml milk
• 150ml log soft rindless goat's cheese , in rough chunks
• freshly grated nutmeg , optional