Juice the limes into a medium mixing bowl. It might seem like there's a lot of juice, but you won’t be sorry. Add the onion and stir to coat. Let sit while you prepare the avocados.
Halve and pit each avocado. Dice flesh and scoop from skins into mixing bowl, tossing with lime juice as you go to prevent browning. Add salt and mash avocado to desired consistency. I like to leave a some pieces diced and mash some more thoroughly for a little textural interest. Add cilantro and jalapeño if using and stir to combine. Serve with tortilla chips and lots of good beer.