Heat 1 tbsp olive oil in a large oven-safe pan over medium heat. Season chicken breasts with salt and pepper and cook until golden and cooked through. Remove and set aside to cool slightly before shredding.
In the same pan, add the remaining olive oil, leek, onion and celery. Cook slowly for 8–10 minutes until softened, sweet and fragrant.
Add garlic and mushrooms and cook for a further 5–6 minutes until the mushrooms have softened and reduced.
Stir through the Creamy One Pot Recipe Base, cream and chicken stock. Add Dijon mustard, rosemary and thyme and stir until combined.
Return the shredded chicken to the pan and stir through the cheese until melted and creamy. Season with cracked pepper and a pinch of salt if needed.
Top with puff pastry, either as whole sheets or layered squares. Brush with egg wash and sprinkle with sesame seeds.
Bake at 180°C for 30–45 minutes, or until the pastry is puffed, golden and crisp.
Allow to rest for a few minutes before serving.
