Clean scallions and trim root. Separate lighter colored bulb and darker green tops. Mince bulbs and thinly slice greens. Mix bulbs with lemon juice, mustard and salt, stirring to dissolve salt. Blend in oil.
Pour dressing into a very wide skillet set over moderate heat. Add leaves and turn to coat with dressing (tongs are most efficient). Cook until leaves almost wilt but stems retain crunch, about 2 minutes. Add scallion greens and toss.
Arrange on a platter, lined up like asparagus. Pour any remaining dressing on top.
Makes four side-dish servings.