Using a cook's knife, finely chop the onion and jalapeño. Add salt and using the flat blade, press down to smash the ingredients. Continue to chop and smash till minced.
Finely chop the cilantro into the minced ingredients.
Prepare the avocados (see notes below), scooping them out into a bowl, and mash with a potato masher to desired consistency.
Combine the minced ingredients with the mashed avocado and squeeze lime over the top. Stir well and serve at room temperature.
How to cut the avocado:
Using a sharp knife, slice the avocado in half, lengthways, rotate the two halves and pull them apart.
Remove the pit by very gently hitting the knife blade into the pit so it sticks, and rotate the pit out.
Scoop into a bowl using a spoon.
Serve guacamole at room temperature.To store: Cover with a thin layer of oil or water (pour this away before serving), and keep in an airtight container in the fridge for up to two days. It will still be safe to eat when it turns dark green, but will taste of unripe avocados when it turns brown.Bring to room temperature before serving.