Place the tomatillos, pasilla pepper, jalapeno pepper, garlic, and onion quarters onto a foil-lined baking sheet.
Roast for 30 minutes, or until the skins are charred and the insides are steamed.
Put the peppers and tomatillos into a plastic zip-top bag and allow to steam for 5 minutes. (You can also use a bowl and then cover it tightly with plastic wrap.)
Peel skins from the peppers and tomatillos using your hands. Then put them into a blender. (For a milder salsa, remove the pepper seeds before blending.)
Add the onion, roasted garlic, cilantro, salt, pepper, cumin, and lime juice to the blender. Cover and blend until smooth.
Pour the salsa into a jar and refrigerate until ready to serve.
Notes
Steaming the tomatillos and peppers after roasting helps release the skin from the fruit. They will still be quite warm after steaming, so be careful while you peel the skins off.