Heat oil in large nonstick skillet over medium-high heat and cook chicken, turning once, until thoroughly cooked, about 8 minutes; remove and set aside.
Cook mushrooms in same skillet over medium-high heat, stirring frequently, until golden, about 5 minutes.
Add Knorr® French Onion recipe mix and water and bring to a boil. Stir in spinach and cook covered until spinach is just wilted and sauce is thickened, about 3 minutes.
Add chicken and simmer until heated through.
