Heat oil in 12-inch nonstick skillet over medium-high heat and cook steak, stirring occasionally, 2 minutes. Season, if desired, with salt and ground black pepper.
Add mushrooms and onion and cook, stirring occasionally, 3 minutes or until tender. Remove steak mixture and set aside.
Bring water to a boil over high heat in same skillet. Stir in Knorr® Asian Sides™ - Teriyaki Rice. Reduce heat to low and simmer covered 7 minutes or until rice is tender. Return steak mixture to skillet; heat through. Garnish, if desired, with chopped green onions.