In a food processor, blend the onion, garlic, and ginger into a paste. (If you don't have a food processor, grate them or mince very finely by hand; just make sure the ginger pieces are very small.)
1 small onion, 2 garlic cloves, 4 thin slices of ginger (2–3 mm)
Add the marinade and 1 tablespoon of soy sauce to the beef; mix well and marinate in the refrigerator for 1 hour.
700 g beef strips, 1 tablespoon light soy sauce
After marinating, sprinkle half of the starch over the beef and use your fingers to coat it well. Let rest for 10 minutes.
140 g potato starch (divided; you can use cornstarch, but it won't be as crispy)
Heat the oil to about 190 degrees.
Oil for frying
After the 10-minute rest and once the oil is hot, add the remaining starch and coat again.
Fry the beef for 1 to 2 minutes (in small batches), then transfer to a paper towel–lined plate.
Once all the beef has had its first fry, return it to the oil for a second fry for 1 more minute.
Pour off all the oil from the pan and return the pan to the stove over medium-low heat.
Add all the sauce ingredients except the water and cornstarch.