To a large ziptop bag, add the steak slices, cornstarch, seal, and toss to coat. Set aside while preparing the sauce.
To a medium saucepan, add 1 tablespoon olive oil, 1 tablespoon sesame oil, garlic, ginger, and heat over medium-high for about 1 minute, or just until garlic is fragrant, taking care not to scorch it.
Add the soy sauce, water, brown sugar, and stir until the sugar has dissolved. Bring to a boil and allow it to boil for about 2 to 3 minutes. Then reduce the heat to low and simmer for about 5 minutes, or until slightly thickened and reduced. Taste the sauce and make any necessary flavor adjustments.
While the sauce simmers, sear the steak. To a large skillet, add the remaining 2 tablespoons olive oil, 1 tablespoon sesame oil, and steak. Cook over medium-high or high heat until steak is cooked through, about 5 to 7 minutes, flipping the pieces intermittently to ensure even cooking.
Add the soy sauce mixture, noting it will bubble up. Toss meat to coat it evenly with the sauce. Allow the sauce to simmer for about 3 minutes at medium-high heat before reducing heat to low and simmering for 3 to 5 minutes, or until sauce has reduced and thickened to taste. Give mixture a final stir to coat all pieces evenly.
Evenly garnish with the green onions, sesame seeds, and serve immediately. Beef is best warm and fresh but will keep airtight in the fridge for up to 5 days.
