Chicken & ham pie

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