For a milder flavor, remove the seeds and membranes from the serrano pepper before boiling. Roasting the poblano peppers can also be done under the broiler if you don't have a cast-iron skillet. Watch carefully to prevent burning. If you don't have Mexican crema, sour cream can be used as a substitute. Feel free to add other fillings to the enchiladas, such as black beans, corn, or sautéed vegetables.