Using a vegetable peeler, peel the cucumber into long strips, discarding the seedy middle. Place in a large bowl with the onion, beansprouts, mangetout and radishes. Toss together, add the beef and toss again.
In a small bowl, combine the lime zest, juice, sugar and 1 tablespoon water and mix until dissolved. Add the soy sauce, chilli and mint, and mix again. Drizzle over the salad and serve.