Heat 4 tbs oil in a heavy large skillet over medium high. Add chopped onions and stir to coat. Season with salt. Saute 20 minutes stirring occasionally. Reduce heat to medium and cook until onions are very soft, stirring frequently about 10 minutes.
Stir in cinnamon and brown sugar. saute until onions are brown about 5 minutes (may be made 1 day ahead).
Coarsely puree quartered onion, lemon juice, cumin, ginger and paprika in blender. Pour into heavy resealable plastic bag. Add chicken, turn to coat. Refridgerate 30 minutes, up to an hour.
Heat two tsp oil in another large skillet over medium. Add chicken with marinade, cook 3 minutes each side or til brown. Transfer to plate. Add tomatoes and broth to skillet, boil until sauce thickens, about 6 minutes. Return skillet to skillet, spoon sauce over. Reduce heat, cover and simmer 15 minutes. Transfer to plate and tent with foil.
Boil sauce until slightly thickened, about 3 minutes. Stir in cilantro, season with salt and pepper.
Rewarm cinnamon onions, mound couscous on 4 plates. Top with chicken, sauce and onions. Serve.