Chicken and Leek Pasties - River Cottage

Ingredients

• 300g plain flour
• A pinch of sea salt
• 150g chilled unsalted butter, cut into small cubes
• 25g butter
• 2–3 medium leeks (about 500g), trimmed and finely sliced
• 1 teaspoon roughly chopped thyme leaves
• 150ml double cream
• 1 teaspoon English mustard
• About 400g boned chicken thigh and breast meat (or leftover cooked chicken), cut into thick slices
• 1 tablespoon rapeseed or olive oil (if using fresh chicken)
• Sea salt and freshly ground black pepper
• 1 medium egg, lightly beaten with 1 teaspoon milk, for glazing