Season chicken, if desired, with salt and ground black pepper. In 12-in. nonstick skillet, melt 1 Tbsp. margarine over medium-high heat and cook chicken, turning once, 8 minutes or until chicken is thoroughly cooked. Remove chicken from skillet and set aside.
In same skillet, melt 1 Tbsp. margarine over medium-high heat and cook asparagus and onion, stirring occasionally, 4 minutes or until vegetables are almost tender.
Stir in Knorr® Hollandaise sauce mix blended with milk and lemon juice. Add remaining 3 Tbsp. margarine. Bring to a boil over high heat, stirring constantly. Reduce heat to low and simmer, stirring constantly, 1 minute. Pour sauce over chicken and serve, if desired, with hot cooked rice and garnish with lemon slices.