For star anise-poached beef, heat peanut oil in a large frying pan over high heat, add beef fillet and brown well all over (3-4 minutes), then transfer to a plate lined with absorbent paper, set aside. Meanwhile, combine stock, soy sauce and spices in a saucepan, season to taste and bring to the simmer over medium heat. Add beef, reduce heat to low, simmer until medium-rare (12-15 minutes), remove beef (broth can reserved for another use).
Cut cucumber into 5cm-long batons, place in a bowl, combine with 2 tsp sea salt and mix well. Stand for 15 minutes, then rinse under cold running water and pat dry with absorbent paper, set aside. Meanwhile, dry-roast peppercorns in a small saucepan over medium-high heat until fragrant (1 minute), then combine with cucumber, oils and coriander.
Divide cucumber salad among plates, place thinly sliced beef on top, serve immediately.