Trim the ends of the onion, split, and peel. Trim the stem from the serranos and split in half. Split the tomatillos.
For boiling: place the vegetables into a pot and cover them with water. Bring to a boil and cook for about ten minutes, or until tomatillos are fully tender.
For roasting: Preheat oven to 400 degrees. Roast vegetables on a parchment or foil-lined baking sheet for about ten to fifteen minutes, or until tomatillos are tender.
For broiling: Place vegetables on foil lined baking sheet. Broil for five to ten minutes, checking once per minute after five, until vegetables are deeply charred and tomatillos are tender.
Allow vegetables to cool slightly. Add your cooked vegetables, garlic cloves, salt, and cilantro to the jar of your food processor. Pulse until a smooth salsa is formed with no large vegetable chunks. Serve warm or cold, or store in the refrigerator for about ten days.