Heat oven to 400 degrees . Crust: Combine flour
and salt in a food processor
. Drop in pieces of shortening and process in on-and-off bursts till combined. Drizzle with 3 to 4 tbsp ice water; process till crust comes together in a ball. Divide in two, with one portion just slightly larger.
With a floured rolling pin, roll larger dough into a 12-inch circle on a floured sheet of wax paper. Invert into a 101/2-inch cast-iron skillet
, patching any cracks. Roll remaining dough into an 11-inch circle. Refrigerate while making filling.
Filling: Heat oil
in a large nonstick skillet over medium heat. Add potato, onion, celery and carrots. Cook, stirring, 5 minutes.
Blend broth and flour; add to skillet along with thyme, salt and pepper. Simmer 10 minutes. Stir in chicken; remove from heat.
Pour filling into bottom crust. Top with remaining crust. Pinch edge together and tuck down slightly into pan. Cut slits into top crust.
Bake at 400 degrees for 35 minutes until lightly browned and bubbly. Cool 10 minutes.