Season chicken, if desired, with salt and ground black pepper. Melt margarine in large nonstick skillet over medium-high heat and cook chicken until thoroughly cooked. Remove and set aside.
Stir in onion and mushrooms and cook, stirring occasionally, 2 minutes or until onion is tender.
Stir in water and Knorr® Rice Sides™ - Rice Medley and bring to a boil over high heat. Continue boiling over medium heat, stirring occasionally, 10 minutes or until rice is tender. Stir in chicken; heat through.