Slice the onion into 1/2-inch (1 cm) strips. Cut the beef into 2 or 3 pieces each and season lightly with salt and pepper. In a small bowl, mix together all the sauce ingredients until well combined.

Heat a frying pan over medium heat and add the vegetable oil. Sauté the onions until they start to soften, then transfer them to a plate.

Increase the heat to high. Quickly sear the beef on both sides, then return the onions to the pan. Add the sauce mixture and green peas or green beans (if using). Stir-fry until the sauce thickens and everything is glossy. Serve hot.