To prepare crust, lightly spoon 3/4 cup all-purpose flour and whole wheat flour into dry measuring cups; level with a knife. Combine flours, salt, and red pepper in a food processor; pulse 2 times. Add shortening; pulse 4 times or just until mixture resembles coarse meal. Transfer mixture to a bowl. Sprinkle ice water, 1 tablespoon at a time, over flour mixture; toss with a fork until moist and crumbly (do not form a ball). Press mixture gently into a 4-inch circle on plastic wrap; cover. Chill 30 minutes.
Preheat oven to 400°.
To prepare filling, place potato in a shallow microwave-safe bowl; add 1 tablespoon water. Lightly cover with wax paper. Microwave at HIGH 2 minutes or until crisp-tender, stirring once.
Heat a medium saucepan over medium heat. Coat pan with cooking spray. Add leek, celery, and garlic; cook 4 minutes. Add mushrooms; cook 3 minutes. Lightly spoon 1/2 cup flour into a dry measuring cup; level with a knife. Place flour in a medium bowl; add broth, stirring with a whisk. Stir broth mixture into leek mixture; bring to a boil, stirring constantly. Reduce heat, and simmer 8 minutes. Stir in potato, chicken, and remaining ingredients. Spoon into an 11 x 7-inch baking dish coated with cooking spray.
Slightly overlap 2 sheets of plastic wrap on a slightly damp surface. Unwrap and place chilled dough on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll dough, still covered, into a 12 x 8-inch rectangle. Remove top sheets of plastic wrap. Place dough, plastic wrap side up, over hot filling. Remove top sheets of plastic. Tuck in overlapping edges of dough. Cut 3 (1-inch) slits in top of dough. Bake at 400° for 25 minutes or until crust is browned and filling is bubbly. Remove from oven. Let stand 10 minutes before serving. Cut into 6 portions.