Ham, leek & potato pie

INGREDIENTS

• 2 tbsp butter
• 1 large onion , finely chopped
• 3 leeks , finely sliced
• 2 large potatoes , cut into small cubes
• 2 tbsp plain flour , plus extra for dusting
• 300ml chicken or ham stock
• 100ml pot crème fraîche (or use leftover double cream)
• 2 tsp wholegrain mustard
• 200g cooked ham , shredded
• 4 bay leaves , optional
• 375g pack puff pastry
• 1 egg , beaten, to glaze (or use milk)