Preheat your oven to 400°F.
Dice the chicken breast into small bite-sized pieces. Chop the carrot, celery, and onion finely, and measure out the green peas.
In a medium saucepan, add the chicken pieces with a splash of water or low-sodium chicken broth (optional additional liquid if desired) and gently cook until lightly browned.
Add the chopped carrot, celery, and onion to the pan. Sauté until the vegetables begin to soften, about 4-5 minutes.
Stir in the green peas and low-fat milk, letting the mixture simmer for another 2 minutes. If a slightly thicker sauce is preferred, sprinkle in the whole wheat flour gradually while stirring to combine.
Once the mixture has thickened slightly, transfer it into a small baking dish.
Sprinkle the remaining whole wheat flour evenly on top to form a light crust. Alternatively, you may mix it with a few small pieces of cold butter for a crumby topping if desired (ensure added fat content fits within your calorie goals).
Place the dish in the preheated oven and bake for 15-20 minutes, or until the topping is lightly golden and the filling is bubbly.
Remove from the oven, let cool for a few minutes, and serve warm.