In a large, heavy pot or Dutch oven, heat the oil and flour over medium-high heat, and cook, stirring constantly with a whisk, until the mixture darkens to a brown about the color of peanut butter (about 6 minutes). This is called a roux, and it will thicken the gumbo and give it a deep, rich flavor. Add the celery, onion, bell pepper, and garlic, and sauté for about 5 minutes, until the vegetables are tender.

Whisk in the stock, add the ketchup, chopped tomatoes, ham, okra, and salt, and simmer over low heat for about 1½ hours, or until the soup has thickened and the flavors are well combined. Stir in the chicken, crabmeat, and shrimp, and simmer for an additional 10 minutes, until the shrimp are fully cooked and the gumbo is warmed through. Add the lemon juice, hot sauce, and Worcestershire sauce, taste for seasonings, and serve in shallow bowls over hot steamed rice.