Combine the spring onions, ginger, soy sauce, half the peanut oil and vinegar in a bowl and stir throughly to combine. Set aside.

Heat the remaining oil in a frying pan. Season the tuna liberally with salt and black pepper. When the oil is smoking, add the tuna to the pan and sear for two minutes per side, until browned. Remove.

While the tuna rests, add the shitake mushrooms to the same hot pan. Cook for another 2-3 minutes, until lightly browned, then add the bok choy. Cook for another 2-3 minutes, until it's lightly wilted.

Slice the tuna into thick strips. Divide the bok choy and mushrooms between two warm plates. Top with slices of tuna, then dizzle with the ginger-spring onion sauce.