Scoop out the avocados and mash in a medium sized bowl.
Add the onion, cilantro, garlic, and jalapeño. Stir to combine.
Taste, then add the lime juice, salt and pepper to your liking.
Place a pit/seed from the avocado into the finished guacamole. Add more lime juice squeezed over the surface, and seal -- preferably with a tight fitting lid or with parchment paper pressed against the surface . The less air contact with your guacamole the better.