In a large bowl, combine green mango, papaya, string beans and lemon juice. Set aside for 5 minutes. In a wok or frying pan, heat oil, add garlic and fry until golden. Add onions and fry until clear. Remove with a slotted spoon and set aside on absorbent kitchen paper.
To the mango mixture add fish sauce, sugar, dried shrimp, chili, cilantro and dried chili flakes, if desired. Toss gently and chill.
Serve on a bed of 2 Bibb lettuce leaves, with garnishes of tomato halves, peanuts, fried onion and garlic