Heat a large, nonstick skillet over medium-high heat. Place the tomatillos cut side down into the pan, and lay the garlic cloves and jalapeno around them. Cook until the tomatillos are well browned, about 4 minutes. Flip everything over and continue cooking until the tomatillos are very soft.

Place the garlic and tomatillos in the bowl of a food processor, and place the jalapeno in a zip top bag to steam. When the jalapeno is cool enough to handle, peel off the blackened skin. Remove ribs and seeds, roughly chop and add to the food processor.

Add the water, onion and cilantro to the food processor, then pulse until combined. Add more water as needed to reach the desired consistency. Add lime juice and salt to taste.