Cut the beef against the grain into thin strips and transfer to a bowl.
Add all the beef marinade ingredients and mix by hand. Let sit 30 minutes.
Coat the beef strips in cornstarch. Shake any excess off. Pan-fry in batches on medium-high in some oil for about 15 seconds on each side. Set aside.
Add vegetables to the same pan and cook until tender-crisp (about 3-4 minutes). Transfer to the same dish as the beef.
Combine all the stir-fry sauce ingredients except the cornstarch and water.
Turn the heat to medium-high and add the sauce, bringing it to a simmer. Make a cornstarch slurry by combining the cornstarch and water, then whisk it into the sauce. Add vegetables and beef back into the pan, stir to coat, and bring to a simmer. Let the sauce thicken for about 2 minutes.
Serve with fresh rice or noodles, or next to fried rice.