Season chicken, if desired, with salt and pepper. Heat oil in large nonstick skillet over medium-high heat and cook chicken, in batches if necessary, stirring occasionally, until browned, about 3 minutes. Remove and set aside.
Add mushrooms to same skillet and cook over medium-high heat, stirring occasionally, until golden, about 4 minutes.
Stir in chicken, tomato sauce, Knorr® Homestyle Stock Chicken and 1/2 cup (125 mL) water and bring to a boil over medium-high heat. Reduce heat and cook covered, stirring occasionally, until sauce is slightly reduced and chicken is thoroughly cooked, about 9 minutes.