For dressing, zest limes to measure 1 teaspoon zest; juice limes to measure 1/4 cup juice. Place zest and juice in small bowl. Add sugar, salt and oil; whisk until blended. Cover; refrigerate until ready to use.
For salad,cut avocado in half lengthwise; remove seed and cut flesh away from skin. Cut avocado into slices
Arrange lettuce, avocado slices, orange sections and onion on small plates. Drizzle with dressing. Sprinkle with cilantro, if desired.