Put the lentils in a medium saucepan and cover with water. Bring to a boil, then simmer for 15–30 minutes, until the lentils are tender but not falling apart.
Meanwhile, rinse the quinoa under cold running water. Bring about 3 cups of water to a boil in a medium saucepan. Toast the rinsed quinoa in a dry skillet for about two minutes, until it becomes fragrant and nutty-smelling. Add the toasted quinoa to the boiling water, cover and cook for 15 minutes.
Peel the mango, then cut off the cheeks (the fleshy section on each side of the central pit). Carefully (mangoes are slippery!), cut the cheeks into thin slices. Reserve the remaining mango flesh for another use as it's difficult to remove neatly (or use it knowing your salad won't be quite as photogenic).
Drain the lentils, rinse them quickly under hot water. Do the same with the quinoa. Add the lentils and quinoa to a large serving bowl, then toss with the mango, watercress, chicken, mint, olive oil, lime juice, salt, and pepper. Adjust the seasoning if necessary, this salad may need a bit more salad than you'd expect.
Whisk together all the dressing ingredients except the coconut cream (and yogurt, if using) with a fork. Whisk in the coconut cream. Taste for seasoning and heat, you may wish to add slightly more curry paste. Drizzle the creamy korma dressing over the salad just before serving.