Season the fish liberally with salt & pepper. Heat oil in a large sauté pan. Add garlic and cook 1-2 min. until fragrant and golden brown. Add harissa, caraway, paprika, tomato paste and sugar and continue to cook, stirring, for another minute or two, then add tomatoes and bring to a simmer. When the sauce is hot, taste for salt and pepper and adjust as needed. Add the fish to the pan and use a spoon to coat with sauce. Place a tight-fitting lid on the pan, reduce to a low simmer and cook until flesh is opaque and flakes easily, 8-10 minutes or more, depending on thickness.
For the Tahini Sauce: Whisk lemon juice and water into the tahini, followed by the oil and salt. Taste and adjust seasoning as needed.
When the fish is cooked through, transfer to a serving platter, drizzle with the tahini sauce and sprinkle with chopped cilantro. Garnish with cilantro sprigs and lemon wedges or diced preserved lemon if desired.