Add all of the sauce ingredients to a medium mixing bowl and whisk until smooth.
Place the beef strips into a large mixing bowl. Add the egg and cornstarch and mix until evenly coated.
Add the vegetable oil to a large, deep-sided skillet over medium-high heat. Once the oil is hot, cook the beef in batches (to avoid overcrowding the pan) for about 30 seconds per side. Then transfer the beef to a plate and repeat until all the beef is cooked.
Remove some of the oil (or add more if needed) until there is 1 tablespoon of oil in the skillet. Add the bell peppers and onion and cook for 2 minutes. Then add the cooked beef and sauce to the skillet. Stir to combine and cook for 2 more minutes.
Serve warm over cooked rice, garnished with green onions.