Remove the outer husks from 6-8 tomatillos. Rinse the tomatillos and 1-2 serrano peppers well.
Bring 4 cups of water to boil in a pot. Then add the tomatillos and serranos. Boil the tomatillos and peppers on medium heat, uncovered, until tender (about 10 minutes). The skins of the tomatillos and peppers should turn from bright green to dark green.
Remove the pot from the stove, and place the tomatillos and serrano peppers (stems removed) in a blender or food processor. (Reserve the water in the pot to dilute the salsa later, if needed.) Pulse the mixture until smooth.
Add the 1/4 medium white onion and 1 clove garlic (minced) and pulse slightly.
Add the 3 sprigs cilantro and pulse just until combined. Add about 1/4 teaspoon salt (to taste) and pulse once more. If desired, dilute the salsa with 1-2 tablespoons of the reserved water from the pot.
Pour the salsa over chicken enchiladas, shrimp tacos, quesadillas, nachos, or your favorite Mexican dish. You can also serve it as a dip with tortilla chips.