Preheat oven to 375°F. CRUST: Whisk dry ingredients together. With pastry cutter or food processor, mix butter into dry ingredients until coarse meal forms. Mix-in egg just until dough forms. Roll-out dough between two sheets of wax paper, peel off the top, place it back on and peel off the other side. Refrigerate. POT PIE: In heavy ovenproof skillet, melt 2 tbsp butter and sauté carrots, onions, mushrooms, and celery about 5 min. In a separate pan, melt 4tbsp butter. Sauté chopped mushrooms and salt for 5 min. Add potato flour and mix for 2 more minutes. Remove from heat and mix-in water until smooth, then milk and thyme. Bring to a simmer then immediately pour over vegetables in the skillet. Add remaining ingredients. Drape pie crust over pan and tuck in edges. Cut slits all around. Bake 1hr.