The cilantro-lime dressing is key to this delicious salad.
In a small bowl, combine onion with 2 tablespoons lime juice, 1/2 teaspoon sugar, and 1/4 teaspoon coarse salt; toss to combine. Let lime-marinated onion sit at room temperature until it softens and brightens in color, about 1 hour (up to 1 day, covered and refrigerated).
Make dressing: In a blender, combine remaining 1/4 cup lime juice, jalapeno, cilantro, and remaining 1 1/2 teaspoons sugar; season generously with salt. Blend until smooth. With the motor running, add oil in a steady stream through the feeder opening until incorporated.
Just before serving, slice avocados; arrange on a serving plate. Top with onion; drizzle with dressing.
When slicing an avocado, do not peel it; cut in half and remove pit. Using a paring knife, slice the flesh without cutting through the skin. Scoop out slices with a large spoon.