Slice the beef as thinly as possible, across the grain. (It’s easier if you freeze it for 15-30 minute first.) Cut the slices into bite sized pieces. Sprinkle with salt and pepper.
Marinate the beef in ¼ cup soy sauce, 2 T. rice vinegar and 2 teaspoons sugar.
Heat a large skillet over high heat until it smokes, 3-4 minutes. Add 1 T oil and the onions. Stir immediately, then stir every 30 seconds or so until the onions soften and begin to char slightly, 4-5 minutes.
Sprinkle the onions with salt and pepper and remove them; keep the heat high.
Add remaining oil to the pan, then the garlic and 1 T. of the ginger; stir and immediately add the beef. Stir immediately, then stir every 20 seconds or so until it loses its color, just a minute or 2 longer; stir in the onions.
Add the remaining teaspoon of ginger; cook until liquid is evaporated.
Garnish with chopped scallions and toasted sesame seeds and serve immediately over rice.