Put the potatoes in a large pan, then cover with cold water and bring to the boil. Cook for 15 minutes, until tender. Drain and cut in half.
Meanwhile, mix together the garlic, 1 tbsp rice vinegar, the peppercorns and the soy sauce in a small bowl. Set aside.
Mix together the radish, mange tout and cucumber. Drizzle over the remaining rice wine vinegar and season. Toss to combine.
Heat the oil in a large frying pan. Add the tuna steaks and cook for 2 minutes, then turn over. Pour in the Szechuan dressing, and continue to cook the tuna for a further 1 minute for medium-rare, or an extra 2-3 minutes for well-done. Serve the tuna with the pan juices drizzled over and the potatoes and salad on the side.