Preheat oven to 375°F. Place 4 8-ounce ramekins on a baking sheet. Set aside.

Unfold the thawed puff pastry, using a rolling pin to flatten out the seams as needed. Cut into 4 squares to fit the tops of the ramekins. Place in the fridge to keep cool while preparing the filling.

Melt the butter in a medium skillet over medium-high heat. Add the vegetables and sauté until the vegetables are crisp-tender, about 5-7 minutes. Sprinkle in the flour, stirring to coat the vegetables. Cook for 1 minute.

Slowly add the milk and chicken stock, stirring constantly. Continue cooking over medium-high heat, stirring constantly, until sauce comes to a simmer and is thickened. Add diced chicken, then season with salt and pepper to taste and remove from heat.

Divide the filling between the prepared ramekins. Whisk together the egg and water — brush the egg wash around the rim of the ramekins, then place a square of puff pastry on each ramekin, pressing lightly around the rim. Cut a slit in the center of each crust to release steam; brush the tops lightly with the egg wash.

Bake for 20-25 minutes, until puff pastry is golden and filling is bubbling. Allow to cool for about 5-10 minutes before serving.