Preheat oven to 400 degrees F. Put the flour on a baking sheet and cook for about 30 to 40 minutes, stirring occasionally, until the flour is a dark nut brown.
Meanwhile, in a large heavy skillet or Dutch oven, heat the oil with the butter over medium heat. Add the onions, pepper, celery, and garlic. Season with salt and pepper and cook until softened, about 10 minutes. Add the tomatoes, crushing them through your fingers into the pot with their juices and cook for another 5 minutes. Gradually sift the browned flour in the pot until the vegetables are coated, cook for 2 minutes more. Slowly pour in the stock and clam juice, stirring constantly. Add the okra, bay leaves, lemon juice, salt, cayenne, thyme, and crabs. Bring to a boil, lower the heat, cover, and cook for 15 minutes.
Add the shrimp, oysters with their liquid, green onions and parsley; cook until the shrimp turn pink, about 3 minutes. Add the lump crabmeat and season the gumbo, to taste. Serve with rice.