Cut the avocados in half. Remove the pit. Scoop out avocado from the peel, put in a medium bowl. Using a fork, mash the avocados.
Add the onion, cilantro, tomato, lemon juice, jalapeno pepper and salt; mix well. Leave one of the avocado pits in the guacamole to keep it green. Cover with plastic wrap directly on the surface of the guacamole to prevent oxidation from the air reaching it.
Refrigerate for at least 30 minutes to blend flavors. Serve as an appetizer with tortilla chips or as a topping for quesadillas.