In a large saucepan, cover the tomatillos, onion, garlic and jalapeńo with water. Bring to a boil and simmer until the tomatillos lose their bright green color, 8 minutes. Drain, transfer to a blender and let cool.
In a small, dry skillet, toast the cumin seeds over moderate heat until fragrant, shaking the pan, about 2 minutes. Transfer to a spice grinder and let cool slightly.
Grind the cumin to a powder and add it to the blender along with the cilantro and sugar; puree until smooth. Season the salsa with salt and serve chilled.