We can use prawns or prawns raw, cooked or frozen. In any case, start peeling them and book. Remove the segments of the grapefruit peeling it lively, without any white part or internal skin, as explained here. Chop the green onion into thin slices.
Heat some oil in a pan and saute the prawns with a little salt and granulated garlic, until they are ready, watching that they do not pass. Remove. We could also grill them first and then peel them to taste.
Beat the oyster sauce (Asian specialty, you can substitute soy sauce) with water, juice and lime zest, chili to taste and a little oil, until make a dressing. Pour a little over cooked prawns.
Assemble the salad by placing a base of watercress or another green leaf (arugula, canons, spinach …). Spread the grapefruit, the scallions and the prawns. Crown with toasted sesame, chopped fresh cilantro and the rest of the dressing to taste.