Season chicken, if desired with salt and ground pepper. Melt margarine in 12-inch nonstick skillet over medium-high heat and cook chicken, stirring occasionally, until chicken is thoroughly cooked. Remove chicken and set aside.
Stir onion and tomato into same skillet and cook 3 minutes or until tender. Add water and milk and bring to a boil. Stir in Knorr® Pasta Sides™ - Cheesy Cheddar and continue boiling over medium heat, stirring occasionally, 12 minutes or until pasta is tender. Stir in chicken; heat through.