Devein the prawns - make a deep cut along the back of each prawn. Discard the dark intestinal vein, then put the prawns aside.
Trim the spring onions and slice diagonally; peel and grate the ginger; peel and crush the garlic; halve, seed and slice the chilli. Put all into a bowl and put aside.
Peel the mangoes and cut into 5-mm-thick slices.
Mix the tomato paste, soy sauce, sherry and sesame seed oil in a small bowl.
Rinse and dry the lettuce leaves and arrange on individual serving plates. Put aside.
Heat the peanut oil in a wok or large frying pan. Add the spring onion mixture and stir-fry for 1 minute. Add the prawns and continue stir-frying until just beginning to turn pink. Add the mango slices and stir-fry until the prawns turn completely pink and the mango is heated through.
Add the soy sauce mixture and bring to the boil. Season to taste with black pepper, then spoon over the salad and serve.