In a medium saucepan over medium-high heat, add the tomatillos, jalapeños, and garlic. Cover with water and bring to a boil.
Reduce heat to medium and simmer for 10 to 15 minutes, or until the tomatillos are softened and pale green.
Drain the vegetables and transfer them to a food processor.
Add cilantro, onion, and salt.
Blend until smooth or to your desired consistency.