1. Using a very sharp paring knife, peel the grapefruits, carefully removing all of the bitter white pith. Carefully cut in between the membranes to release the grapefruit sections into a bowl. Discard all but 1 tablespoon of grapefruit juice from the bowl. Stir in the pickled cocktail onions and parsley leaves and season with pepper.
2. Pat the sea scallops dry and season them all over with salt. In a large nonstick skillet, heat the 1 tablespoon of olive oil until it is shimmering. Cook the scallops over moderately high heat, turning once, until they are browned and just cooked through, about 4 minutes total. Spoon the pickled-onion-and-grapefruit salad onto small plates and arrange the scallops around the salad. Drizzle with olive oil and serve at once.