Mix the marinade ingredients in a bowl big enough to hold the beef. Add the beef and mix well. You will need to use your hands here and just massage everything together.
Let sit for 30 minutes, or longer if desired.
Heat a skillet or non-stick pan over medium-high heat.
Remove the beef from the marinade.
Strain the marinade Into a saucepan over medium heat and bring to a simmer. Slowly stir the dissolved cornstarch into the marinade a little at a time (may not need it all) until the marinade thickens to a smooth consistency then remove and set aside.
Add the oil to the skillet or pan and sauté the onions for 2- 3 minutes. Add the beef and just pan fry/stir fry the meat until it’s slightly brown on both sides (your pan should be hot enough so that the meat sizzles as it touches the pan). Don't put too much meat into the pan or you will end up with a lot of the meat juice leaking out.
Serve immediately. Spoon the sauce over the beef and garnish with scallions.