Cut beef into matchstick-size strips about 1/16 inch thick and no more than 2 inches long; put into a large bowl. In a 4- to 5-quart pan over high heat, bring broth to a boil. Pour over beef and stir. For rare beef, drain at once; for medium, let stand 5 minutes, then drain (save broth for other uses).
In bowl, mix meat with lime juice, lemon juice, and onion; add chili flakes and salt to taste. Cover and chill until cold, at least 1 hour or up to 6 hours, stirring occasionally.
Meanwhile, working over a bowl, with a small, sharp knife, cut off and discard peel and white membrane from grapefruit. Cut between fruit segments and inner membranes to release fruit into bowl. Squeeze juice from membranes into bowl; discard membranes.
With a slotted spoon, lift beef mixture from bowl and mound equally on plates. Lift grapefruit segments from juice and arrange equally alongside; reserve juice for other uses. Add equal portions of watercress. Spoon beef marinade over fruit and greens. Add fish sauce or more salt to taste.
Nutritional analysis per main-dish serving.