Preparation

• If the preserved lemon is whole, cut it into 4 to 6 wedges. Rinse the wedges and remove the seeds, if necessary. Using a spoon or paring knife, separate the pulp from the peel. Finely chop the pulp and set aside. Finely chop half of the pieces of peel, then slice each of the remaining pieces into 3 or 4 strips and set aside.
• Set a medium-mesh sieve over a large bowl. Working over the sieve, run your fingers through the tomato halves to release the seeds and press the liquid through the sieve, discarding the seeds. Remove the sieve and set a box grater into the bowl. Working with one tomato at a time and holding each in the palm of your hand, slowly and carefully grate the tomatoes on the large holes of the grater until the skin peels back and the soft flesh has been grated into the bowl; discard the skin.
• Add the lemon pulp and chopped peel, parsley, cilantro, garlic, cumin, paprika, 1/4 tsp. salt, and a few grinds of pepper to the grated tomatoes. Add 2 Tbs. of the oil and 2 Tbs. water and mix well. One by one, add the fish fillets to the marinade, and turn to coat each one before adding the next. Cover and refrigerate, turning the fish once or twice, for 30 minutes
• Meanwhile, bring a 4-quart pot of lightly salted water to a boil. Add the carrots and potatoes and simmer until tender but not falling apart, 6 to 10 minutes. Drain in a colander and let cool slightly.
• Heat an 8-inch skillet over medium heat. Add the bell pepper and 1/4 cup water, cover, and steam until beginning to soften, about 5 minutes. Remove from the heat; set aside.
• Coat the bottom of an 11- to 12-inch tagine with the remaining 2 Tbs. oil. Evenly arrange the carrots and potatoes over the bottom of the tagine, and drizzle with 1/4 cup water. Separate the onion slices and scatter them on top. Arrange the fish over the vegetables and spread any remaining marinade lightly and evenly over all. Arrange the bell pepper strips on top and dot with the olives and strips of lemon peel.
• Put the tagine over medium-low heat and cover with the lid, propping a wooden spoon between the base and the lid to keep it from sealing. Gently simmer, nudging the fish and vegetables from time to time to prevent sticking and swapping the spoon position halfway through, until the fish is opaque throughout (use a paring knife to check), 30 to 45 minutes. Drizzle in a few tablespoons of water as necessary during cooking to keep the sauce loose, or remove the lid at the end of cooking to evaporate and thicken the sauce if it’s watery. Serve.