Place serranos, cilantro, onion, lime juice, salt and garlic in a food processor (or blender), and pulse on low speed for 5 seconds.
Add tomatoes, and pulse on low speed until desired consistency. (these proportions make a medium heat salsa. Cut down on serranos if you are not a chilihead, or add more for insanity.)
Notes
2 serranoes makes a medium heat, adjust as desired.