Make the salsa: In a large saucepan, combine tomatillos, onion, bell pepper, chiles, and garlic. Add 1/2 cup water and a pinch of salt. Cover and bring to a boil. Remove from heat once tomatillos are tender and set aside until cooled. Puree in a blender along with cilantro and season with salt to taste.
Make the enchiladas: Preheat the oven to 350°F. Season the chicken with salt (and other seasonings of choice).
In a skillet, heat oil. Once oil is hot, carefully dip each tortilla in and out of oil until softened. Place chicken in center of each tortilla, roll up, and arrange in a baking dish. Gently pour tomatillo sauce over enchiladas, sprinkle with cheese evenly, and bake until cheese has melted. Serve with onion and sour cream, if desired.