Preheat oven to 400˚F.
In a skillet, heat the oil over medium heat. Add the onion and cook for 5 minutes. Add 1/2 the beans and mash into a chunky paste. Stir in the reserved liquid.
Grease a heavy, ovenproof skillet. In a bowl, combine the corn, parsley and remaining black beans. Place a tortilla in the skillet and spread with 1/3 cup of the mashed black bean mixture. Top with 2/3 cup corn-bean mixture and 1/2 cup cheese. Press to compress the layers. Repeat with the remaining tortillas, mashed black beans, corn-bean mixture and cheese.
Bake until cheese is melted and the casserole is heated through, about 30 minutes. Transfer to the broiler and cook for 1 minute. Serve with the enchilada sauce.