Place the tomatillos, serrano chiles, chopped onion, and garlic cloves in a saucepan and cover with water.
Bring to a boil, then reduce heat and simmer until the tomatillos change color but do not burst, about 5–8 minutes.
Use a slotted spoon to transfer the cooked vegetables to a blender.
Blend until smooth, adding chicken broth a little at a time to reach a medium sauce consistency, then add the cilantro and pulse a few seconds to combine.
Heat the tablespoon of vegetable oil in a clean saucepan over medium heat until shimmering.
Pour the blended sauce into the hot pan and cook, stirring, until warmed through and slightly thickened, about 3–4 minutes.
Reduce heat to a bare simmer and stir in the crema Mexicana; heat gently without boiling until combined and warmed through.
Remove from heat, season with kosher salt to taste, cover, and keep warm until ready to use on enchiladas.
