The acid in the tomatillos will keep your guacamole bright green even if not serving right away.
In a food processor, combine avocados, tomatillos, onion, jalapeno, cilantro, 2 teaspoons salt, 1/4 teaspoon pepper, and 1/2 cup water. Puree until smooth. Serve immediately or refrigerate in an airtight container up to 3 days.
For a milder guacamole, remove the seeds of your jalapeno before using.