Preheat the oven to 400°F. In a skillet, heat the oil over medium heat. Add the onion and cook for 5 minutes. Add 1 1/2 cups black 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, bean mixtures and cheese. Bake until the 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.