In a large heavy-bottomed pot, combine 0.75 cup of all-purpose flour and 0.75 cup of vegetable oil over medium heat. Stir constantly for 20-30 minutes, watching the roux darken to a rich chocolate color.

Add 1 large diced yellow onion, 2 chopped celery ribs, and 1 chopped green bell pepper to the roux. Cook for 5-7 minutes until the vegetables soften and become fragrant, stirring frequently. Then, stir in 4 minced garlic cloves and continue cooking for an additional minute.

Add 12 ounces of sliced andouille sausage to the pot. Cook for about 5 minutes, stirring occasionally, until the sausage is lightly browned.

Gradually whisk in 5 cups of seafood or chicken stock. Add 1 cup of diced tomatoes and 2 bay leaves, and stir to combine. Bring to a gentle simmer over low heat for about 5 minutes.

Stir in 2 tablespoons of Cajun seasoning, along with salt and black pepper to taste. Allow the gumbo to simmer uncovered for 45-60 minutes, occasionally stirring.

In the last 10-15 minutes of cooking, gently fold in 1 pound of raw shrimp and 8 ounces of lump crab meat. Stir carefully, and cook until the shrimp turn opaque.

Remove the bay leaves from the pot, taste, and adjust seasoning if necessary. Garnish with 0.5 cup of chopped green onions just before serving. Serve hot over cooked white rice.