In a small bowl, combine the soy sauce and sugar
Place the steaks in a small ziplock bag. Pour the soy sauce marinade over the steak and seal the bag. Place in refrigerator and marinate for 1 hour.
While the steak is marinating, make the sauce by combining cornstarch, white wine vinegar, and water in a bowl. Stir until smooth.
Remove steak from the marinade. Keep the marinade for the sauce later.
Heat oil in a skillet over medium-high heat. Add garlic and cook until fragrant, then set aside.
Add steak and cook for 3 minutes, stirring occasionally. Remove from heat and set aside.
Add the sliced onions, bell peppers, and reserved garlic to skillet. Cook for 2 minutes, or until vegetables are soft, then reduce heat to medium-low.
Return steak to skillet and add sauce and leftover marinade. Cook until the sauce thickens, about 2 minutes. Serve over rice.