Marinate steak in soy sauce and corn starch. Allow to sit at least 10 minutes.
Meanwhile, prep other ingredients.
Turn on wok or skillet on high. When pan is hot, add oil. When oil is shimmery, add flank steak and stir fry for 3-5 minutes until beef is just cooked through. Then remove from heat.
Add oil to the skillet/wok, and when oil is shimmery, add in garlic, bell pepper, onions, and red jalapeno. Stir fry for 3-5 minutes, until vegetables are slightly softened.
Add in beef and stir fry together. Add in oyster sauce and sugar. Stir fry another minute. Taste for seasoning. Add in basil and stir fry until basil has just softened.