Combine the enchilada sauce, minced onion and the cocoa. Simmer 10 minutes; set aside. Combine 2 cups of the cheese, the beans and the canned chilies. Soften tortillas by wrapping in aluminum foil and heating in a 300° oven for 20 minutes. Spread a thin layer of enchilada sauce in the bottom of a 9 x 13-inch pan. Fill each warm tortilla with some filling; roll to close. Place, seam side down, in the pan. Spoon remaining sauce over enchiladas and top with remaining shredded cheese. Bake in a preheated 325° oven for 25 minutes. Garnish with plum tomatoes and green onion. Makes 8 enchiladas.