In a Dutch oven or a heavy-bottomed pot, melt the butter over medium heat. Add the flour and cook, stirring constantly with a whisk or wooden spoon and breaking up any lumps as you stir. You want the roux to be a lush golden brown, which takes about 8 minutes. Carefully add the onions, bell peppers, celery, and garlic, stir well, turn the heat to low, and cover. Stir again after 10 minutes and continue cooking until the vegetables are tender, about 20 minutes. Add the tomatoes and clam juice (or broth) to the pot, stir, and then add 1 teaspoon each of the black pepper, cardamom, coriander, and ginger, as well as the cayenne and ½ teaspoon of the kosher salt. Bring to a boil, then reduce to a simmer over medium-low heat and cook until the tomatoes have fallen apart and the liquid has reduced by about half, about 30 minutes. Meanwhile, season the shrimp with the remaining black pepper, cardamom, coriander, ginger, and kosher salt. Add the shrimp to the pot and stir. Cook the shrimp until they are cooked through, 4 to 5 minutes. Serve with rice or flatbreads.