Place the tomatillos in a medium saucepan, cover with water and bring to a boil over high heat. Boil until they turn brownish-green and are soft, 5 to 7 minutes. Remove with a slotted spoon and immediately place into a bowl of ice water.: The water should roar as it comes to a boil and the tomatillos will bob and rattle slightly, releasing a faint vegetal steam that smells bright and tart. This step loosens their skins and softens the flesh so the blender can make a silky base. If your pot is crowded, the tomatillos may steam instead of boil, which leads to uneven softness, so give them room and a steady boil.
Place the drained tomatillos, Hatch chiles, cilantro, onion, garlic, lime juice, salt and cumin in a blender and puree until smooth. Place in an airtight container and refrigerate for at least 1 hour before serving. The salsa can be refrigerated for up to 4 days.: You will notice a color shift to a muted green and the tomatillos should collapse slightly when pressed with a spoon, giving a soft, yielding texture. The boiling time concentrates flavor and mellows raw tartness. Avoid overcooking them for much longer than 7 minutes because they can become mealy and lose the bright notes that define a great salsa.
Remove with a slotted spoon and immediately place into a bowl of ice water: The shock of cold water clamps down on the heat, preserving the vivid green hue and stopping carryover cooking, which keeps the texture lively. You should hear a soft plink as they land in the ice bath and the temperature shock will firm the skin. Skipping this step can leave the tomatillos dull and overly soft, which changes the final mouthfeel.
Place the drained tomatillos, Hatch chiles, cilantro, onion, garlic, lime juice, salt and cumin in a blender and puree until smooth: As the blender starts, you will smell the bright citrus and warm cumin lifting up, and the mixture will thicken from a loose slurry to a smooth, glossy sauce. Pureeing integrates the aromatics so each spoonful is balanced. Pulse first to break down larger pieces, then blend until smooth; overblending at very high speed can warm the salsa and fade the fresh green color.
Place in an airtight container and refrigerate for at least 1 hour before serving: Chilling helps the flavors meld, softening edges and allowing the heat from the Hatch chiles to spread evenly through the salsa. The aroma will deepen and the texture will settle to a cohesive sauce. If you serve it immediately, it will taste bright but less integrated, so give it an hour when possible for the best results.
The salsa can be refrigerated for up to 4 days: Stored cold in an airtight container, the salsa maintains its quality for several days and often tastes better after a day as the flavors continue to develop. Expect the color to slightly dull with time, and always smell and taste before serving. If you notice any off aromas or separation that does not remix, discard it.