Place the couscous in a large heatproof bowl. Bring 1 cup water to a simmer in a small saucepan; pour over the couscous, cover and let stand until the water is absorbed, about 10 minutes.
Meanwhile, grate the zest of 1 lemon, juice 1 1/2 lemons and cut the remaining 1/2 lemon into 4 wedges. Fluff the couscous and add 2 tablespoons olive oil, the lemon juice, bell pepper, cucumber, tomatoes, shallot and parsley; season with salt and pepper. Refrigerate while the fish cooks.
Mix the za'atar, lemon zest and 1/2 teaspoon each salt and pepper in a small bowl. Rub onto the flesh side of the fish. Heat 1 tablespoon olive oil in a large nonstick skillet over high heat. Add 2 fish fillets skin-side down, pressing gently to prevent them from curling. Cook until the edges turn opaque, about 4 minutes. Carefully flip the fish and cook until just opaque throughout, about 2 minutes; remove to plates. Add the remaining 1 tablespoon olive oil and repeat with the remaining 2 fish fillets.
Top the fish with parsley and drizzle with olive oil. Serve with the couscous salad and lemon wedges.
You don’t need much olive oil to cook this low-cal snapper, but a drizzle at the end adds extra flavor!