Place tomatillos, onion, garlic, and jalapeño (if using) on a baking sheet. Roast for 20 minutes, or until the tomatillos are softened and slightly charred.
Let cool slightly, then transfer the roasted vegetables to a blender. Add cilantro, salt, and pepper.
Blend until smooth, or to your desired consistency. Add a little water if needed to thin it out.
Taste and adjust seasonings as needed.
Notes
Serve immediately or chill for later. This salsa verde is great with chips, tacos, or enchiladas.