Heat the oven to 200°F. Pat the fish fillets dry and season all over with 1/2 tsp. salt and 1/4 tsp. pepper. Put the flour in a wide, shallow dish and lightly dredge the fish in the flour, shaking off any excess.
Heat the oil in a 12-inch nonstick skillet over medium-high heat until shimmering hot. Arrange the fish skin side down in the pan so the fillets fit without touching. Cook undisturbed for 4 minutes. With a spatula, peek under a fillet to see if the skin is golden-brown and crisp. If not, cook 1 minute more. Flip the fillets and cook until lightly golden-brown and just cooked through, an additional 1 to 2 minutes. Transfer to a wire rack set over a baking sheet and keep warm in the oven.
Reduce the heat to medium, add the cumin seeds to the skillet, and cook, stirring, until fragrant, about 10 seconds. Add the tomatoes, 1/2 tsp. salt, and 3 Tbs. water and cook until they become saucy, 3 to 4 minutes. Stir in the chickpeas and cook until heated through, about 3 minutes. Add the preserved lemon (or lemon zest) and chopped cilantro. Season to taste with salt and pepper.
Scrape a fork through the couscous to fluff it, and then divide it among 4 dinner plates. Place the fish skin side up on the couscous. Spoon on the chickpea mixture and garnish with the harissa and a scattering of cilantro leaves. If desired, drizzle extra-virgin olive oil over the fish at the table.