Preheat oven to 375°F (190°C).
In a large bowl, whisk together milk, bone broth, cornstarch, and chicken salt blend until smooth.
Add raw chicken cubes, frozen vegetables, and diced onion to the glass containers. Mix well to combine.
Divide the milk mixture evenly among four 600 ml oven-safe containers.
Top each container with a square of puff pastry. Trim excess if necessary and press edges to the sides of the containers. Cut a small slit in the top of each to allow steam to escape.
Brush the puff pastry with beaten egg.
Place the containers on a baking sheet to catch any potential overflow.
Bake for 25-30 minutes, then check. If the pastry is golden but the filling isn't bubbling, cover loosely with foil and continue baking for another 5-10 minutes.
The pies are done when the pastry is golden brown and the filling is bubbling. The internal temperature of the chicken should reach 165°F (74°C).
Let stand for 5 minutes before serving. Caution: Containers will be hot!