Thinly slice the flank steak on a 45-degree angle, about 1 cm thick (slightly less than 1/2 inch). Use a very wide piece of flank steak, so you can slice them against the grain into long 6-inch (15cm) slices.
Add the beef to a large bowl, along with the sugar, cornstarch, five spice powder, garlic powder, onion powder, white pepper, salt (if using), neutral oil, oyster sauce, sweet bean sauce, light soy sauce, sesame oil, and water. Mix well, cover, and refrigerate overnight, or at least 4 hours. During that time, soak bamboo skewers in water (minimum 1 hour; or you can use metal skewers).
Line a baking sheet with foil and a wire rack. Thread the pieces of beef on the skewers so they lay flat. Place them on the wire rack about 1 inch (2.5cm) apart. For crispier results, use two wire racks and two sheet pans or cook in two batches.
Position an oven rack at the top-most level, closest to the heating element. Turn your oven on broil (high). Broil the skewers for 3-4 minutes on each side, watching them closely so they don’t burn. (Ovens vary!) Garnish with toasted sesame seeds if desired, and serve.
Notes
Serves 6 as a main course or 12 as an appetizer. Nutrition information is for appetizer portion. If you want to make these beef on a stick a little more festive, cut your beef into 1/2 inch thick chunks instead, and mix an 8-ounce can of pineapple chunks in with the beef to marinade. Then alternate threading the beef and pineapple on the skewers!