Preheat your oven to 400°F.
Dice the chicken breast into bite-sized pieces, and season lightly with salt and pepper if desired.
Chop the carrot, celery, and onion into small, uniform pieces.
Heat the olive oil in a medium saucepan over medium heat. Sauté the onion, carrot, and celery until they soften, about 3-4 minutes.
Add the diced chicken to the pan and continue to cook until the chicken is lightly browned on all sides.
Sprinkle the whole wheat flour over the mixture and stir well to coat evenly. This will help to thicken your sauce.
Pour in the low-sodium chicken broth and low-fat milk, stirring continuously until the mixture begins to thicken slightly.
Stir in the frozen peas and allow the filling to simmer for another 2-3 minutes, ensuring all components are heated through.
Transfer the filling into a small oven-safe baking dish.
Roll out the whole wheat puff pastry to cover the top of the filling. Trim any excess and place the pastry over the dish, pressing gently at the edges.
Bake in the preheated oven for 15-20 minutes until the pastry is golden and the filling is bubbling.
Remove from the oven, let cool slightly, and serve warm for a comforting meal.