Step 1: Preheat Oven : Preheat oven to 400°F (200°C).

Step 2: Cook the Chicken : Heat butter and olive oil in a large skillet over medium heat. Season chicken with salt and pepper. Cook until lightly golden and just cooked through. Remove and set aside.

Step 3: Cook the Leeks : In the same skillet, add leeks and cook gently for 5–7 minutes until softened. Add garlic and cook for 30 seconds.

Step 4: Make the Sauce : Stir in flour and cook for 1 minute. Gradually add chicken stock, stirring constantly until thickened. Stir in cream, thyme, and Dijon mustard.

Step 5: Combine Filling : Return chicken to the skillet. Simmer gently for 3–5 minutes until the sauce is creamy. Taste and adjust seasoning.

Step 6: Assemble the Pie : Spoon the filling into a pie dish or oven-safe skillet. Lay puff pastry over the top, trimming excess. Crimp edges and cut small slits for steam. Brush with beaten egg.

Step 7: Bake : Bake for 35–40 minutes until pastry is golden and puffed.