In a large saucepan, combine the milk, chicken broth, and stock powder. Heat gently over medium heat until just starting to simmer. Add the chicken and thyme. Cover the saucepan and let it simmer on medium-low heat for 15 minutes. Ensure it does not boil to prevent the milk from curdling. Remove the chicken, shred or dice it (a little undercooked inside is fine). Keep the poaching liquid covered and set aside.

Melt the butter in a large pot over medium-high heat. Add the minced garlic and chopped onion, cooking for 2 minutes. Stir in the thyme, chopped carrot, and celery, and cook for an additional 3 minutes or until the carrots begin to soften. Pour in the white wine, stirring and scraping the bottom of the pot to deglaze, cooking until the wine has mostly evaporated, about 1.5 minutes. Sprinkle in the flour, stirring continuously for 1 minute to create a roux. Gradually add half of the reserved poaching liquid, stirring until the flour is fully incorporated and the mixture thickens. Pour in the remaining milk-broth mixture, followed by the grated parmesan and black pepper. Stir to combine. Add the shredded chicken and frozen peas, mixing well. Once the mixture starts to steam, cook for 3 more minutes, stirring regularly to thicken the sauce. Remove from heat and spoon the mixture into oven-proof pots, either 4 large or 6 small ones. Let the pots 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, making sure they are about 2.5cm / 1″ wider than the pot rims. Preheat your oven to 180C/350F. Brush the edges of the pots with the whisked egg. Place the puff pastry rounds on top, folding down the edges over the sides. Brush the pastry with the egg wash and make a 2cm / 2/3″ slit in the center with a small knife. Bake for 35-40 minutes until the pastry is a deep golden brown. Serve immediately!