If you are cooking the pot pies immediately, preheat the oven to 375.
Start by making the potatoes: put the potatoes into a saucepan, cover them with water and bring to a boil. Simmer until the potatoes are tender, about 15 minutes. Drain and return to the pot.
Add 4 tablespoons of butter and 1 teaspoon salt. Mash with a potato masher and then add the pepper and the milk. Stir in half the scallions and set aside.
In another large saucepan, melt 2 tablespoons of butter over medium high heat. Add the onions and stir until they are starting to turn tender, about 3 minutes.
Whisk in the flour, followed by the chicken broth. Simmer for 5 minutes.
Add the carrots, celery, 1 teaspoon salt and the sage. Simmer for another 5 minutes.
Add the milk and the chicken and simmer for another 5 minutes.
Spoon the chicken mixture into one large casserole or individual casseroles. Top with the mashed potatoes, spreading them to the sides of the casserole. At this point you can put them in the fridge if you are baking them later, just let them come to room temperature before you bake.
Place the casserole on a rimmed baking dish and bake for 30 minutes. Cool slightly, garnish with remaining scallions and serve!
Notes
Chicken: You can use leftover chicken, buy a rotisserie chicken at your supermarket, or poach a chicken breast specially for this recipe.
Mashed Potatoes: The recipe below includes how to make mashed potatoes from scratch, but you can also pick up a pre-made container of mashed potatoes (look in the dairy section of your supermarket, heat them up according to the package directions, and they will work just fine.
Poultry Seasoning: This is a combo of spices including dried sage, thyme and a few others, and you can find it in the spice aisle of your supermarket. If you want to try making your own, here is a great poultry seasoning recipe for you to check out!