Chicken and Ham Pie

Name: Chicken and Ham Pie
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Cuisine: British
Ingredients: drizzle of Olive Oil; 600g / 1.3lb boneless skinless Chicken Thighs ((see notes)); 1/2 tsp EACH: Salt, Black Pepper; 1 medium Carrot, (finely diced); 1 medium rib of Celery, (finely diced); 1 small/medium Leek, (dark/firm part removed then finely diced); 3 tbsp / 45g Unsalted Butter; 3 tbsp Plain Flour; 480ml / 2 cups Chicken Stock, (use low-salt if sensitive to salt); 120g / 1/2 cup full fat Creme Fraiche, (at room temp (see notes)); 2 tbsp finely diced Fresh Parsley; 1 tbsp Dijon Mustard; 150g - 200g / 5oz - 7oz Cooked Ham, (roughly diced (see notes)); 1x 320g/11oz sheet of Shortcrust Pastry; 1x Egg, (beaten)