Coat the flank steak pieces in cornstarch. Let the meat sit in the cornstarch for 10 minutes.
Allow the vegetable oil to heat before laying meat in a single layer in the pan. You may need to work in batches for this and add more vegetable oil. Brown both sides of meat until cooked through, about 1 minute on each side.
Once the meat is cooked, set it aside and whisk the fish sauce, light brown sugar, honey, lemon juice, garlic powder, ground ginger, and cayenne pepper into the skillet. Stir frequently to prevent burning.
Once the sauce starts to boil, turn the heat down to let it simmer until it thickens, stirring occasionally.
When the sauce has thickened, add the flank steak to the sauce and then add in the scallions. Mix together well and take off heat.
Plate and serve over rice.