1. Place the lemon zest and thyme in a small bowl; toss. Sprinkle the chicken with salt and pepper on both sides. Sprinkle half of the lemon-and thyme mixture evenly onto one side of each cutlet. Heat the olive oil in a large nonstick skillet over medium-high heat; cook the chicken, herb side down, turning after 2 minutes. When the chicken is golden and cooked through (about 4 minutes), transfer to a cutting board. Cover chicken, and keep warm.
2. Bring 1/2 cup water to a boil in a small saucepan; gradually stir in the couscous. Remove pan from heat; cover and let stand 5 minutes. Fluff with a fork. Cover couscous, and keep warm.
3. Sauté the zucchini and squash (in the same skillet used for the chicken) over medium heat, stirring frequently, until golden and tender (about 4 minutes). Stir in the remaining lemon-and-thyme mixture, chicken broth, and couscous.
4. Spoon the couscous mixture evenly among 4 plates; top each with a chicken cutlet. Garnish with thyme.
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