To be strictly Thai you should leave out the watercress and increase the quantity of herbs. If you can't find green mangoes, or prefer to eat ripe ones, you can use 1 ripe mango and 1 tart green ...

In a very large bowl, combine the green mango, cucumbers ... Add the dressing and toss well. Transfer the salad to plates and mound the crab on top. Garnish with the scallions and serve.