Peel and remove the husks from the tomatillos and rinse under cool water. In a medium pot, add 8 cups of water and bring it to a boil. Add the onion, serranos, jalapenos, and tomatillos in the boiling water,  then bring to a simmer and cook until the tomatillos are slightly yellow and fairly soft but not falling apart, 8 to 10 minutes. Remove from heat a let it cool slightly.

Transfer the jalapenos, serrano, smashed garlic, and kosher salt to a blender or food processor; reserve the cooking water. Blend for 5 seconds. Add the tomatillos and blend to a rough texture. Blend in the cilantro and blend until smooth, adding a few tablespoons of the cooking water if the salsa is too thick. Taste for seasoning and adjust if necessary. Let it sit for 30 minutes to let the flavors develop. Stir in lime juice if using.