You can prepare the chicken and vegetables one day, and assemble and bake the potpie the next day. I purchased chicken in a package containing 4 & 1/2 lbs. of bone-in split chicken breasts with skin on each. I only used two halves which I browned in the oil and butter, and I then added two cups of water and seasoning (poultry seasoning and salt and pepper) and simmered covered for thirty minutes.
I peeled three carrots,sliced them and boiled them until tender.I sliced one stick of celery and half an onion and sauteed them for five minutes. Combine all vegetables and set aside.
When the chicken was done I removed the bones from the meat and pulled it apart into chunks, and I saved the broth. When cool skim off the fat. Measure out about 12 ounces, and add the jar of chicken gravy, adjust seasonings adding tarragon to taste and thicken with the cornstarch which I had stirred into 2 ounces of milk.
I stored the chicken in a covered dish, put the vegetables and gravy in the refrigerator also, and removed the puff pastry from the freezer and kept it in the refrigerator until I needed it.
Place four ovenproof bowls on a baking sheet, spoon chicken and vegetables and gravy into each, not more than 3/4's-full. Preheat the oven to 375 degrees with shelf at half-level.
On a floured surface open pastry and roll gently, cut four rounds which are two inches larger across than each bowl. Place a pastry round on each bowl securing it to the rim, pinching it, and brush with beaten egg-water wash. Bake 30-35 minutes until browned and nicely puffed-up. Let rest briefly before serving.