London Broil has come to mean two things over the years:
marinated & broiled flank steak that's sliced thin
and also designates thick cuts of steaks found
in the meat department.
Of these cuts, top round, sirloin and shoulder,
top round is the most tender out of the three.
All of them need to be marinated as they are
not the most tender of cuts from beef.

Simply marinate a one to two pound top round
London Broil or flank steak in Master Choice
Zesty Italian Dressing overnight or for at least
several hours.  Drain.
Broil the steak 1" from the flame for 4-5 minutes
on each side.
Transfer steak to cutting board.  Allow steak
to rest 5 minutes.
Using a sharp knife, cut the London Broil into
 very thin slices by cutting against the grain
and at an angle at which you knife seems
almost parallel to the cutting board.
Serve over Caesar Salad or which a cold
plate of vine-ripe tomatoes and roasted potatoes.