In a large pot, combine the whole chicken with diced onions, carrots, celery, garlic, thyme, bay leaf, and enough water to submerge everything. Bring to a gentle simmer over medium heat and cook for approximately 1 hour.

Carefully remove the chicken from the pot and set it aside to cool slightly. Strain the leftover stock through a fine mesh sieve to collect around 500ml of liquid.

Once cooled, discard the skin and bones from the chicken and shred the meat into bite-sized pieces.

In a spacious pan over medium-low heat, melt the butter and whisk in flour to form a roux, cooking for about 2-3 minutes until light golden. Gradually add the chicken stock while whisking to avoid lumps, and cook until thickened.

Fold the shredded chicken into the sauce, coating it well. Stir in any additional vegetables if desired.

Pour the filling into a deep pie dish and roll out the puff pastry to fit. Cover the dish, crimping the edges to seal, and cut slits in the top for steam.

Preheat the oven to 180°C (350°F) and brush the pastry with egg wash. Bake for about 30-35 minutes until golden.

Let the Chicken Pie rest for 5 minutes before serving.