In small bowl, beat together eggs, water, and mustard until well blended.  In 10-inch non-stick omelet pan over medium-high heat, heat margarine until just hot enough to sizzle a drop of water.  Pour in egg mixture.  Mixture should set immediately at edges.  With an inverted pancake turner, push cooked portions from the edges of the pan toward the center so uncooked portions can spill onto the hot pan surface.  Tilt the pan as necessary, keeping the bottom covered with egg.  When top is thickened and no visible liquid egg remains, fill one-half of the omelet with vegetables.  With pancake turner, fold omelet in half.  Invert to serve. Spoon sauce over omelet, if desired.