For the seared tuna, cut out a circle of tuna with a 10cm/5in cutter. Season well with salt and freshly ground black pepper and drizzle with olive oil.
Heat a griddle pan until smoking hot. Add the tuna and griddle for 1-2 minutes on each side. Remove from the heat and set aside.
For the sautéed sweet potatoes, place the sliced sweet potatoes into a pan of boiling salted water and boil for 4-5 minutes. Drain well.
Heat the olive oil in a small frying pan and add the blanched sweet potato. Season, to taste, with salt and freshly ground black pepper and fry for 3-4 minutes on each side, or until tender and golden-brown.
For the charred spring onions, rub the spring onions with olive oil and season with salt and freshly ground black pepper. Heat a griddle pan until smoking hot, add the spring onions and griddle until charred on all sides.
To serve, pile the sautéed sweet potatoes onto a serving plate. Top with the seared tuna and garnish with the rocket.
