In a large saucepan, combine the milk, chicken broth, and stock powder. Heat gently over medium heat until it starts to simmer. Add the chicken and thyme, cover, and let it simmer on medium-low for 15 minutes. Avoid boiling to prevent the milk from splitting. Once done, remove the chicken, shred or dice it, and set it aside. Keep the poaching liquid covered.

In a large pot, melt the butter over medium-high heat. Add the minced garlic and chopped onion, cooking for about 2 minutes until fragrant. Then, add the thyme, carrots, and celery, cooking for an additional 3 minutes until the carrots soften.

Pour in the white wine, stirring and scraping the bottom of the pot. Cook for about 1.5 minutes until most of the wine has evaporated. Next, add the flour and stir for 1 minute to combine.

Gradually add about half of the reserved poaching liquid, stirring until the flour is fully incorporated and the mixture thickens. Then, add the remaining milk broth, grated parmesan, and black pepper, stirring well.

Incorporate the shredded chicken and frozen peas into the mixture. Heat until steaming, then let it cook for 3 minutes to thicken the sauce, stirring regularly. Remove from heat.

Spoon the filling into oven-proof pots, either 4 large or 6 small, and allow them to cool in the fridge for at least 30 minutes.

While the filling cools, take the puff pastry out of the freezer to partially thaw. Use a bowl to cut out rounds from the pastry that are about 2.5 cm / 1 inch larger than the pots.

Preheat the oven to 180°C/350°F. Brush the edges of the pots with the whisked egg. Place the puff pastry rounds on top, folding down the edges. Brush the pastry with egg and cut a small slit in the center, about 2 cm / ⅔ inch long.

Bake in the preheated oven for 35 to 40 minutes, or until the pastry is deep golden brown. Serve immediately!