Peel the shallot, cut in half and finely chop, mix well with the creme fraiche, oil, vinegar and sugar in a bowl. Season with salt and pepper and set aside.
Quarter and core the melon, cut off the skin and divide the flesh into narrow wedges. Halve, peel and stone the avocados and cut into narrow strips.
Wash and dry the lettuce leaves and arrange on plates with the melon and avocados. Then spread the prawns on top and drizzle the dressing over the salad.
Wash the coriander, pluck the leaves, garnish the salad with it and serve.