In a bowl, make a marinade by whisking together the soy sauce, lemon juice and sugar. Add the beef to the marinade and let stand while you cook the onions.
In a nonstick frying pan, heat up a bit of oil over high heat until hot and shimmery. Add the ginger and onion and cook, turning down the heat and stirring as needed, until soft.
Push the onions and ginger to the side. Turn the heat up and add the beef, reserving the marinade, in a single layer if possible. Cook, undisturbed, until seared. Flip and sear the other side.
Make a sauce by mixing the cornstarch and 1/3 cup water into the remaining marinade.
Add the sauce and cook until the beef is tender and the sauce bubbles, thickens, and becomes glossy. Season with a generous amount black pepper and enjoy.