Chicken and Leek Pie

Name: Chicken and Leek Pie
Prep time: 15 min
Cook time: 1 h 20 min
Total time: 1 h 20 min
Yield: 4 portions
Ingredients: 450 g liquid chicken stock; 80 g onions; ½ tsp fine sea salt; 3 black peppercorns; 1 garlic clove; 2 sprigs fresh thyme; 1 bay leaf; 600 g chicken breasts; 200 g leeks; 200 g carrots; 30 g unsalted butter; 30 g plain flour; 250 g milk; ¼ tsp fine sea salt; 2 pinches ground black pepper; 1 tsp Dijon mustard; 50 g white wine; 375 g puff pastry, ready-made; 1 egg yolk