Pat the tofu dry on kitchen paper and cut into 1 cm (½ inch) cubes. In a small bowl, mix together the ginger, soy sauce, garlic and vinegar. Add the tofu to the bowl and toss the ingredients together. Set aside to marinate for 15 minutes.
Lift the tofu from the marinade with a fork, drain it and reserve the marinade. Heat the oil in a frying pan and gently fry the tofu pieces for about 3 minutes until golden. Drain and keep them warm.
Add the spring onions and cashews to the pan and fry quickly for 30 seconds. Add the mango slices to the pan and cook for 30 seconds, until heated through.
Pile the lettuce on to serving plates and scatter the tofu, spring onions, mango and cashews over the top. Heat the marinade juices in the pan with the water, pour the mixture over the salad and serve immediately.