Bring 1 1/4 cups water to a boil. Stir in the couscous, lemon peel and 2 tablespoons olive oil; season with salt and pepper. Cover and let stand for 5 minutes before fluffing with a fork. Stir in 1/4 cup parsley just before serving.
Meanwhile, in a large nonstick skillet, heat the remaining 2 tablespoons olive oil over medium-high heat. Season the chicken with salt and pepper. Working in batches, add the chicken to the pan and cook until golden on one side, about 5 minutes; turn and cook for 1 minute more. Transfer to a plate. (You may need to add another tablespoon of olive oil between batches.) Add the onion and garlic to the pan, partially cover and cook, stirring occasionally, until soft and golden, about 6 minutes.
Return the chicken to the pan and stir in the tomatoes, olives, lemon juice and 1/2 cup water. Simmer until the chicken is cooked through and the sauce thickens, about 5 minutes; stir in the remaining 1/2 cup parsley. Serve with the couscous.
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