In a large skillet, heat the olive oil. Add the onion and cook over moderate heat, stirring, until softened, 4 minutes. Add the garlic and cook for 1 minute. Add the chicken and cook over moderately high heat, stirring occasionally, until browned all over, 3 minutes. Stir in the soy sauce and sesame oil, season with salt and pepper and stir-fry until the chicken is cooked through, 3 minutes longer. Remove from the heat and stir in the lemon zest and lemon juice. Season with salt, pepper and soy sauce. Transfer the chicken to a platter, top with the sliced scallion and serve with rice.