Chicken & ham pie

Name: Chicken & ham pie
Prep time: 25 min
Cook time: 45 min
Total time: 1 h 10 min
Cuisine: British
Ingredients: 6 boneless skinless chicken breasts, cubed; 3 carrots, chopped; 3 potatoes, peeled and cubed; 2 celery stalks, chopped; 2 tsp chopped fresh thyme or 1 tsp dried; 850ml hot chicken stock; 50g butter; 2 onions, chopped; 50g plain flour; 300ml milk; juice of 1 lemon; 2 tbsp chopped parsley; 4 thin slices ham, cut into strips; 500g pack shortcrust pastry; 1 egg, beaten