In a medium bowl, combine soy sauce and cornstarch. Add beef and toss to coat.
In a nonstick skillet over medium-high heat, add water, peppers and garlic. Cook and stir for 3 to 4 minutes until peppers are crisp-tender. Remove from skillet.
In same skillet, heat oil until hot. Add half of beef and cook for 2 to 3 minutes. Remove and add remaining beef and cook for 2 to 3 minutes, adding more oil if needed.
Return all beef to skillet and add sauce mixture, peppers, and pineapples. Heat through.