Cook the pastry lids – Preheat the oven to 200°C (400°F) (180°C/350°F fan-forced). Cut the puff pastry into four even squares and arrange them on a baking tray lined with baking (parchment) paper. Score the puff pastry squares diagonally with a butter knife, every 1.5 cm (½ inch). Using a pastry brush, brush the pastry with the whisked egg. Sprinkle it with salt and sesame seeds. Bake for 15 minutes, or until golden and crispy. Remove from the oven and set aside. 

Cook the filling – Meanwhile, heat the olive oil in a large pan over medium heat. Add the leek and cook for 2–4 minutes until softened. Add the garlic and cook, stirring, for a further 30 seconds. Add the chicken and cook for 5–6 minutes, until browned.

Add the mushrooms and cook, stirring, for 3–4 minutes, until softened.

Add the thickened cream, dijon mustard, chicken stock powder and thyme. Cook for 5 minutes or until the sauce has thickened. 

Remove from the heat, stir the cheese through and season with salt and pepper to taste. 

Allow the pie filling to stand, uncovered, for 5 minutes (it will thicken as it cools).

Assemble and serve – Remove the thyme sprigs and ladle the filling into four individual small bowls. Top with the puff pastry “lids” and serve immediately.