To prepare crust, lightly spoon 3/4 cup flour into dry measuring cups; level with a knife. Combine flour, cornmeal, sugar, and salt in a large bowl; cut in shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle surface with ice water and vinegar; toss with a fork until moist and crumbly (do not form a ball). Press mixture gently into a 6 x 4-inch rectangle on heavy-duty plastic wrap; cover with additional plastic wrap. Chill 15 minutes.
Preheat oven to 400°.
To prepare the filling, drain clams in a colander over a bowl, reserving juice. Set clams aside. Combine the reserved clam juice, 1 1/2 cups milk, and thyme in a large saucepan; bring to a boil. Add potato and onion. Bring to a boil; cook 2 minutes. Drain potato mixture in a colander over a bowl, reserving cooking liquid. Set potato mixture aside. Return cooking liquid to pan over medium heat. Lightly spoon 1/3 cup flour into a dry measuring cup; level with a knife. Combine 1/2 cup milk and flour in a small bowl; stir with a whisk. Stir flour mixture into cooking liquid in pan. Stir in wine and bacon. Bring to a boil; cook 5 minutes, stirring frequently.
Remove from heat; add potato mixture. Partially mash with a potato masher. Stir in clams, celery salt, and next 4 ingredients (celery salt through spinach). Spoon mixture into an 11 x 7-inch baking dish coated with cooking spray. Roll dough into a 12 x 8-inch rectangle. Remove 1 sheet of plastic wrap; fit dough on top of clam mixture, pressing to edge of dish. Remove top sheet of plastic wrap. Cut 5 slits in top of crust to allow steam to escape. Bake at 400° for 30 minutes or until golden brown and bubbly around edges. Let stand 10 minutes.