Halve the carrot lengthwise and thinly slice on an angle. Thinly slice the onion. Cut the pak choy into thirds, keeping the leaves and stems separate. Crush or finely chop the garlic. Coarsely chop the coriander, including the stems.
Step 2 — Make stir-fry sauce
Put the oyster sauce, a pinch of chilli flakes (you can omit the chilli when cooking and serve it at the table), half the cornflour*, 2 tbs water, 1 tbs soy sauce and 1 tsp white vinegar in a bowl and whisk to combine. Bring a medium saucepan of water to the boil for the noodles.
Step 3 — Stir-fry beef
Separate the beef stir-fry. Heat 2 tsp oil in a large deep frypan over high heat until very hot (beef stir-fry is best with hot, fast cooking; get the pan very hot before adding the beef and stick to the short cooking time). Stir-fry the beef for 1-2 mins until browned. Remove from the pan.
Step 4 — Stir-fry vegetables
Heat 2 tsp oil in the same pan over high heat. Stir-fry the carrot and onion for 2-3 mins until just starting to soften. Add the pak choy stems and garlic and stir-fry for a further 1 min.
Step 5 — Cook noodles
Meanwhile, cook the noodles in the pan of boiling water for 2 mins or until tender. Drain, then rinse under hot water to prevent the noodles from sticking.
Step 6 — Get ready to serve
Whisk the stir-fry sauce until combined, add to the vegetables and cook, stirring, for 30 secs. Add the beef and any resting juices, pak choy leaves, noodles and coriander and cook, tossing, for 1 min or until combined and the pak choy leaves are wilted. Divide the stir-fry among bowls. Scatter with the remaining chilli flakes, if desired, to serve.