Heat the oil in a large heavy pot or Dutch oven over medium heat. Whisk in the flour and cook, stirring constantly, to make a fragrant, dark chocolate-colored roux, 25 to 30 minutes.
Add the onions, bell peppers, celery, Essence, salt, and pepper. Cook, stirring often, until the vegetables soften, about 4 minutes. Stir in the stock and crabs. Bring to a boil. Reduce the heat to medium-low. Simmer, uncovered, stirring occasionally, for 1 1/2 hours.
Meanwhile, heat a large skillet over medium-high heat. Add the chopped and sliced sausage and cook, stirring often, until browned, about 5 minutes. Remove from the heat.
Stir the shrimp, crawfish, and cooked sausage into the gumbo. Cook over medium-low heat until the seafood is firm, about 10 minutes. Remove from the heat and let stand for a few minutes. Skim any fat from the surface.
Stir the green onions and parsley into the gumbo. Spoon the rice into soup bowls and ladle the gumbo on top. Serve hot.
Yield: 8 to 10 servings