Heat oil in a wok or pan over medium-high heat.
Add garlic and cook for 10 seconds (do not brown!).
Add dark sauce and white sugar, stir until sugar is dissolved, and bring to a boil for about 30 seconds.
Add prepared flank steak and toss in sauce, allowing the sauce to reduce and thicken slightly with the beef for about 30-60 seconds.
Add sesame oil and scallion tops, toss beef with the sesame oil and scallion tops, and cook for about 15-20 seconds longer.
Serve.