Dice the Red Onion (1/3 cup) and transfer to a medium mixing bowl.
Roll the limes across your cutting board to activate the juices. Halve the Lime (2), and squeeze the juice directly into the bowl.
Finely mince the Serrano Chili (2). For less heat, remove the seeds and white pith before mincing.
Transfer the minced peppers to the bowl.
Chop the Fresh Cilantro (1/2 cup).
Cut the Avocado (4) in half and remove the pits.
Scoop the flesh out of the avocados into the bowl. Mash gently with a fork to combine with the onions, peppers, and lime juice.
Fold in the chopped cilantro. Season to taste with Salt and Pepper (to taste). Serve immediately or cover tightly with plastic wrap and store in the refrigerator.