Put the mustard powder and sugar in a large bowl and stir in the vinegar until smooth. Whisk in the oil until thoroughly blended. Season with salt and freshly ground black pepper.
Add the cubed beef, sun-dried tomatoes, spring onions and chives to the bowl, and stir to coat with the dressing. Cover and chill for at least 30 minutes (or up to 8 hours).
While the beef is marinating, cook the rice in a saucepan of boiling water for 8-10 minutes, until just tender. Drain well and spread out on a tray to cool completely.
Transfer the cooled rice to a bowl and stir in the celery, carrot, zucchini, mushrooms and parsley. Cover with plastic wrap and chill until required.
About 10 minutes before serving, remove the marinated beef and the rice salad from the refrigerator. Line four plates with radicchio or other salad leaves. Add the beef mixture to the rice salad and gently stir until well mixed. Spoon the salad onto the bed of leaves and serve.
This recipe also works well with brown rice, although the cooking time will be longer.
Per serving: 667 calories, 48 g protein, 27 g fat (8 g saturated fat), 56 g carbohydrate (7 g sugars), 4 g fibre, 266 mg sodium