Heat 1 tablespoon oil in a large skillet over medium heat and saute onions, garlic, salt, and white pepper for 3 minutes. Add tomatoes and basil to onion mixture and cook, stirring, for 3 minutes. Stir in stock and 18 turns black pepper; simmer 2 minutes. Season with sugar if acidic, and cook 1 minute. Keep warm until ready to serve.
Meanwhile, heat 2 tablespoons oil in a very large skillet. Season chicken with salt and pepper and add to skillet, in batches if necessary. Cook for a couple minutes on each side, or until cooked through. Remove chicken from skillet when done; transfer to a plate and cover to keep warm if not serving immediately.
Serve chicken with couscous on the side and with the tomato sauce spooned over the chicken.