Put tomatillos, onion, garlic, and chile in a 2- to 3-qt. saucepan with 2 cups water. Cover and bring to a boil over medium-high heat, then reduce heat and simmer until tomatillos start to split, 10 to 12 minutes. Uncover and set aside to cool about 45 minutes (don't drain).
Reserving liquid, transfer ingredients from pan to a blender with a slotted spoon. Add 1/4 cup tomatillo water to blender and purée until smooth.
Add cilantro and pulse three or four times. Pour salsa into a bowl and stir in lime juice and salt to taste.
Make ahead: Up to 4 days, chilled airtight.