01 - Preheat your oven to 400°F (200°C).
02 - In a large skillet or Dutch oven, melt 2 tablespoons of unsalted butter over medium heat. Add the diced yellow onion, carrots, and celery; sauté for 5–6 minutes until they begin to soften. Stir in the minced garlic and cook for an additional minute until fragrant.
03 - Sprinkle 1/3 cup of all-purpose flour over the sautéed vegetables. Stir constantly for 1–2 minutes, ensuring the flour is lightly golden and well incorporated.
04 - Gradually whisk in 2 cups of low-sodium chicken broth and 1 cup of whole milk. Bring the mixture to a simmer, stirring often, until it thickens to a creamy consistency, which typically takes about 3–4 minutes.
05 - Add 2 cups of shredded or diced cooked chicken, 1 cup of frozen peas, 1 teaspoon of fresh thyme leaves (or 1/2 teaspoon dried thyme), 1/2 teaspoon of salt, and 1/2 teaspoon of black pepper to the thickened base. Stir thoroughly to combine all ingredients, then remove the filling from the heat.
06 - Pour the prepared chicken pot pie filling evenly into a 9x13-inch baking dish or a similar ovenproof dish.
07 - In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 tablespoon of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt.
08 - Add 6 tablespoons of cold, diced unsalted butter to the dry ingredients. Using a pastry cutter or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs.
09 - Stir in 3/4 cup of cold buttermilk and 1 tablespoon of chopped fresh parsley (if using) just until the biscuit dough comes together. Be careful not to overmix, as this can lead to tough biscuits.
10 - Drop 6–8 generous spoonfuls of the biscuit dough evenly over the hot chicken pot pie filling in the baking dish.
11 - Bake the chicken pot pie in the preheated oven for 25–30 minutes, or until the biscuit topping is golden brown and the filling is visibly bubbling.
12 - Remove the pot pie from the oven and allow it to rest for 10 minutes before serving, which helps the filling set and prevents burns.