For the Red Chile Sauce, toast cumin in a large skillet over low heat, until fragrant, 2 minutes. Transfer to a cup; set aside. Add ancho chiles to same skillet and heat over low heat until chiles become pliable and fragrant, 3 to 5 minutes. Transfer to a large bowl and cover with hot tap water; let stand 15 minutes, until very soft. Drain liquid from chiles. Discard stems and seeds; chop chiles.

Heat oil in a small Dutch oven or medium saucepan over medium heat. Add cumin with onions, garlic, serrano and oregano and cook until vegetables soften, 5 minutes. Stir in chopped ancho chiles, broth, coffee and salt. Bring mixture to a boil; sprinkle in masa harina. Reduce heat to medium-low and simmer mixture, partially covered, 1 hour. Transfer mixture to blender and puree. Return Chile Sauce to Dutch oven and keep warm on low heat.

Heat oven to 350 degree F.

Wrap warm tortillas in foil. Spoon 1 cup warm Red Chile Sauce into the bottom of a 13x9-inch baking dish. Place 1/4 cup cheese in center of one warm tortilla and roll up (keep remaining tortillas wrapped). Place enchilada seam side down in prepared dish. Repeat with remaining tortillas and cheese. Pour remaining 2 cups Chile Sauce over enchiladas. Cover dish with foil and bake until cheese has melted and Chile Sauce is hot, about 15 minutes. Garnish top with onion, if desired. Serve immediately. Makes 4 servings.