Preheat the oven to 450 degrees . In a large saucepan, bring the chicken broth to a boil. Place the couscous in a medium bowl and drizzle the broth on top; wrap tightly with plastic wrap and let stand for 10 minutes before fluffing with a fork.
In the same saucepan, combine the lentils and garlic with water to cover by 1 1/2 inches. Bring to a boil, then lower the heat and simmer until the lentils are tender, about 20 minutes; drain. Discard the garlic, if desired. Stir the lentils into the couscous; reserve the saucepan.
Meanwhile, season the chicken with salt and pepper. In a large bowl, whisk together 2 tablespoons olive oil, the ginger and cinnamon; add the chicken and toss. In a large, ovenproof skillet, heat 1 tablespoon olive oil over medium heat. Add the chicken and cook until golden, 6 to 8 minutes; flip, then transfer the skillet to the oven and roast until cooked through, about 15 minutes.
In the reserved saucepan, heat the remaining 2 tablespoons olive oil over medium heat. Add the onion and cook, stirring, until golden, about 10 minutes. Stir into the lentil mixture with any reserved pan juices from the chicken; season with salt and pepper. Serve with the chicken.