Heat the oil in a large heavy saucepan. Add the celery, onion and green peppers and cook over moderately low heat, stirring, until wilted, about 10 minutes. Add the tomato, scallions and garlic and cook over moderately high heat, stirring, for 2 minutes. Add the fish broth and saffron, season with salt and pepper and bring to a simmer. Add the chiles, thyme, lime zest and allspice and simmer over low heat for 10 minutes.