Press garlic through a garlic press. Mix with soy sauce, balsamic vinegar, and sugar to make a seasoning sauce.
Cut the spring onion lengthwise into very thin slices. Cut the beef fillet into thin slices and toss with the seasoning sauce. Cut the chicory into individual leaves, cutting any large leaves in half.
Heat the oil in a wok or large pan over high heat. Fry the beef for 1.5 minutes, turning occasionally, then remove from the pan.
Add the chicory to the pan and fry for 1 minute, turning constantly.
Return the beef to the pan, toss everything together, and cook for 1 minute. Season with salt and freshly ground pepper. Scatter the spring onion slices on top.