Heat a dash of olive oil in a large frying pan and cook the chicken pieces over a high heat for 3 minutes. Add the leek, garlic, soft cheese, mustard, stock cube and 200ml/7fl oz boiling water. Pour the mixture into a large ovenproof dish and lay the puff pastry sheet over the top. Brush with milk.
Bake for 35 minutes or until the top is golden-brown and the pie is cooked through. Serve.