Thaw the chicken and pat it dry. Melt the butter in a large saute pan over medium to medium-high heat. Add the chicken sprinkle generously with salt and pepper. While the chicken is cooking, chop the vegetables. Slice the onion, celery and carrots. Dice the potatoes into small chunks.
Once the chicken is nice and brown on the bottom, turn it over and sprinkle it with flour and sage. Stir it well, until the flour is absorbed by the butter and the chicken is well-coated.
Stir in the chicken stock and reduce the heat a bit. Let it cook for a few minutes then cut the chicken into smaller chunks if you like. Add the vegetables and let it simmer for a bit, until the potatoes and carrots start to get tender.
While the chicken and vegetables cook, butter a pie pan and press in the bottom crust. Sir in the peas. Then spoon the chicken/vegetable mixture into the crust. If you have lots of filling left, you can make another pot pie or save the filling for a later date.
Add the top crust, seal the edges and cut some slits for ventilation. Bake at 350 for 45 minutes to an hour, until the gravy is nice and bubbly and the crust is nicely browned.