In a medium bowl, combine the parsley with the chopped cilantro, lemon juice, paprika, ginger, saffron, and olive oil. Add the shrimp, toss to coat with the marinade and refrigerate for 2 hours.

Meanwhile, in a medium saucepan, combine the tomatoes with the garlic and cumin and season with salt and pepper. Simmer over moderate heat, stirring occasionally, until the tomatoes are broken down and slightly thickened, 10 minutes.

Spread the sliced carrots in a large enameled cast-iron casserole or Dutch oven and season lightly with salt and pepper. Add the onion, potatoes, and bell peppers, lightly seasoning each layer with salt and pepper. Spread the tomato sauce on top. Cover and simmer over moderately low heat until the vegetables are just tender, about 20 minutes.

Add the preserved lemon and olives to the casserole and arrange the shrimp on top in a single layer. Season with salt and pepper. Cover and cook over moderately low heat until the shrimp are pink and curled, about 3 minutes. Transfer the shrimp to a plate. Raise the heat to moderate and cook the tagine uncovered, stirring once or twice, until the juices have evaporated, about 5 minutes; don't let the vegetables burn on the bottom. Transfer the vegetables to a bowl and top with the shrimp. Garnish with cilantro sprigs and serve.

The tagine can be prepared through Step 3 and refrigerated overnight. Rewarm before proceeding.