Seafood Gumbo

1 cup unsifted all-purpose flour
1 cup vegetable oil
3 cups diced celery (about 6 stalks)
3 cups diced onion (about 3 medium)
3 cups diced green bell pepper (about 3 large)
7 cups reduced-sodium canned chicken stock
3 cups (about 1 1/2 pounds) okra, sliced horizontally into 1/2-inch slices (optional)
1 pound smoked ham, diced
1/2 pound crab meat
2 (28-ounce) cans crushed tomatoes, juice reserved
1/2 cup freshly chopped parsley leaves
1 tablespoon fresh lemon juice
1 tablespoon sugar
1 teaspoon dried oregano leaves
1/2 teaspoon dried thyme leaves
1/2 teaspoon Old Bay seasoning
1/2 teaspoon salt
1/4 teaspoon ground black pepper
1/4 teaspoon ground red pepper
2 pounds medium fresh or frozen shrimp, peeled
1/2 teaspoon gumbo file powder
9 cups cooked rice
Hot pepper sauce, to taste

In a 12-quart heavy kettle or Dutch oven, heat flour and oil over medium-low heat. Cook, stirring frequently, until flour browns to a dark, mahogany color, being careful not to burn, for about 40 to 50 minutes. Stir in the celery, onion and bell pepper. Cook, stirring occasionally, until vegetables are soft, for about 30 minutes. Add chicken stock, okra, ham and crabmeat; cook for 40 minutes more. Stir in the tomatoes and juice, parsley, lemon juice, sugar, oregano, thyme, Old Bay seasoning, salt, black pepper and red pepper. Cook for 30 minutes. Stir in the shrimp and file powder; cook until shrimp have cooked through, for about 15 minutes. Serve warm over rice with hot pepper sauce or freeze for as long as 2 months. Makes 18 1 1/2 cup servings.