Halve and pit the avocados and scoop the flesh into a medium bowl with a spoon. Add the lime juice and 1-1/2 tsp. salt and mix well, using the spoon or a potato masher to break the avocado into small chunks. Stir in the onion, tomato, cilantro, and serrano. Cover with plastic until ready to serve.
Make Ahead Tips
Guacamole should be made no longer than 1 hour before serving.