Preheat oven to 400 degrees.
Place bottom crust in pie pan. Pierce several times with a fork and place into oven. Bake for 10 minutes while you prepare the filling.
In a large pot, melt butter over medium-high heat. Add onion, celery, carrot and salt. Saute until onions are translucent and vegetables are tender.
Sprinkle flour over veggies, add seasonings and garlic and cook for 1-2 minutes.
Gradually whisk in milk, heavy cream and Better Than Boullion. Bring to a slow boil.
Simmer gently over medium heat until sauce thickens (5 minutes). Turn heat to low and add chicken, peas and thyme.
Pour chicken mixture into prepared pie crust. Place top crust over chicken mixture and crimp edges. Cut a vent hole in the center of the top crust.
Brush beaten egg onto crust. Bake 30-35 minutes or until crust is golden brown.
Cool for 5 minutes before serving.