Cook the potatoes in a saucepan of boiling water for 12–15 minutes until soft.
Meanwhile, brush the oil all over the tuna steaks. Heat a griddle pan until smoking hot. Add the tuna and cook for 2–3 minutes on each side for medium-rare. While the tuna is cooking, heat the soy sauce, vinegar and sugar in a small saucepan for about 2 minutes until warmed and slightly syrupy. Mix together the crème fraîche and wasabi.
Drain the potatoes and mash until smooth, then stir through the wasabi mixture. Season well. Spoon the mash on to warmed serving plates along with the tuna. Drizzle over the warm sauce and serve with some steamed sugar snap peas, if you like.