Place sliced steak in a bowl. Sprinkle with the cornstarch and mix until meat is coated evenly. Set aside.
In a large pan or wok, stir-fry vegetables in olive oil until cooked. You may have to cook in several batches depending on the quantity. Once done, add all cooked vegetables to a large bowl and set aside.
Add about 1 tablespoon of olive oil to pan. Add garlic and stir-fry over medium heat for 30 seconds. Add soy sauce, ginger, brown sugar and water. Stirring constantly, bring to a slight boil, about 2 minutes, before removing from heat to a small bowl and setting aside.
Add 1 tablespoon of olive oil to empty pan and stir-fry beef over medium heat until meat is browned. Add salt and pepper. Do not overcook; the thinner the beef, the quicker it will cook.
Leave meat in pan and add cooked vegetables, toss and then add sauce. Continue to cook over medium heat while stirring to combine everything, about 1 minute.
Serve immediately, alone or with steamed rice.