Place the tomatillos, serrano chiles, coarsely chopped onion, and peeled garlic in a saucepan and cover with water; bring to a boil.
Reduce heat to a simmer and cook until the tomatillos change color and soften but do not burst, about 5–8 minutes.
Use a slotted spoon to transfer the cooked vegetables to a blender; reserve some cooking liquid if needed.
Blend the vegetables until smooth, adding chicken broth or some of the reserved cooking water as needed to reach a medium sauce consistency, then add the cilantro and pulse a few seconds more.
Heat 1 tablespoon vegetable oil in a saucepan over medium heat.
Pour the blended sauce into the hot oil and cook a few minutes until warmed through and slightly thickened, stirring occasionally.
Reduce heat to a bare simmer and stir in the crema Mexicana; heat through without boiling.
Remove from heat, season with kosher salt to taste, then cover and keep warm until ready to use for enchiladas.