Starting at the top, with a very sharp knife, cut down into each avocado until you hit the pit. Cut completely around the avocado, top to bottom, touching the pit the entire time.
Pull the two halves of the avocado apart and remove the pit with a spoon.
Use the spoon to scoop the avocado out of the skin. (Some people like to score the avocado with a knife before scooping.) Make sure to gently scrap the skin to get the dark, creamy "good stuff."
Place 2/3 to 3/4 of the softest avocados in a mixing bowl.
Add the juice of one lime immediately to prevent browning.
Using a fork, roughly mash avocados.
Place the remaining firmest avocados on a cutting board or plate and chop into 1/4 to 1/2-inch pieces. Add these to the mashed avocado in the bowl and mix well.
Add red onion, tomato and cilantro to the mashed avocado. Stir gently to combine.
Taste. Add juice of the second lime and kosher salt to taste.
Stir slightly until all ingredients are incorporated.
Stir in Serrano chilies, to taste.
Serve immediately with your favorite tortilla chips as a dip or use as a topping for tacos, nachos, or other Mexican dishes.
Serving Tip: For an extra flavor boost, refrigerate the guacamole for 30 minutes before serving to allow the flavors to meld together.