Put the potatoes in a large pan, cover with cold water and bring to the boil. Cook for 15 minutes, until tender. Drain and cut in half.
Meanwhile, in a small bowl, mix together the garlic, 1 tbsp of the rice vinegar, the crushed Szechuan peppercorns and the soy sauce. Set aside.
Make the salad: mix together the radish, mange tout and cucumber. Drizzle over the remaining rice wine vinegar and season with freshly ground black pepper. 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 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.
Cook's tip: you could also flake the cooked tuna and toss it together with the salad and potatoes to enjoy cold.