In a small bowl, mix the soy sauce, water, cornstarch and sugar. Place ¼ cup mixture in a large bowl and set aside the small bowl.
In the large bowl with the ¼ cup mixture, add 1 Tbsp oil and garlic. Add beef, mix to coat, and let beef sit in mixture for 15 minutes.
Heat a large skillet over medium high heat and add half the beef mixture. Stir-fry and cook beef for 1-2 minutes until it’s no longer pink. Remove from pan. Add remaining beef to skillet, cook for 1-2 minutes and remove from skillet.
Then, heat remaining oil in the skillet on medium-high heat. Add onions and cook until tender. Add vegetables of choice and cook until tender.
Add remaining soy sauce mixture to skillet and stir. Bring to a boil. Cook and stir 1-2 minutes until sauce is thickened. Return beef to pan, heat thoroughly and serve with rice.