1. Heat 2 tablespoons of the oil in a large skillet on medium to high heat. Once the pan is hot, add the beef strips and sear quickly on all sides. Remove from pan and place in a bowl. Cover with foil.
2. Return the pan to the heat and add 1 tablespoon of oil. Saute the onions for 4 minutes then add them to the beef strips.
3. Return the pan to the heat and add the last tablespoon of oil. Saute the mushrooms and peppers together for 4 minutes.
4. Return the beef and onions to the pan with the peppers and mushrooms. Add the Teriyaki sauce and continue to cook until beef has reached desired doneness (5-8 minutes). Serve over cooked rice.
5. Alternative method: If you don't feel like messing with slicing fresh vegetables, just pick up a frozen package of Asian vegetables and prepare according to package instructions. Prepare the beef as described in step 1. Combine with sauce and finish as described in step 4.