Enchiladas have always been thought of as Mexican "soul food" because of their bright, reassuring flavors. These enchiladas suizas, or Swiss-style enchiladas, are simple to make and boast a creamy, savory texture.
Roast the tomatoes and chiles on a baking sheet 4 inches below the broiler until they're darkly roasted with blackened and blistered spots, 5—6 minutes per side. (If using canned tomatoes, drain the tomatoes and set aside. Toast the chiles in a small dry skillet over medium heat, turning regularly, until they're soft and splotchy-black, about 5 minutes.)
Dice the onion, reserving a few rings for garnish. Heat the oil in a large, heavy-bottomed pot over medium heat; cook the diced onions, stirring regularly, until golden, about 7 minutes. Meanwhile, remove the tomatoes and chiles from the oven and cool.
Working over your baking sheet, pull off and discard the blackened tomato skins and, for round tomatoes, cut out the hard “cores” where the stems were attached. Transfer tomatoes and chiles to a food processor or blender, along with all the juices on the baking sheet. Blend to a smooth purée.
Add the puréed tomatoes and chiles to the onions and cook, stirring, until they darken in color and thicken to the consistency of tomato paste, 7–8 minutes. Stir in the broth, partially cover, and simmer 15 minutes. Whisk in the sour cream and season with 1 teaspoon of salt.
Lay the tortillas out on a baking sheet (2 sheets if you have them, for more even heating), and lightly brush both sides of the tortillas with lard or oil. Bake at 350 degrees F just long enough to warm through and soften, about 3 minutes. Stack the tortillas and cover with a towel to keep them warm and pliable.
Spread about ¼ cup of the sauce over the bottom of four to six 9-inch individual oven-proof serving dishes, or smear about 1 cup of the sauce over the bottom of a 13x9-inch baking dish. Working quickly while the tortillas are still pliable, roll a portion of the chicken into each tortilla, then line them up in the dishes, seam-side down. Cover with the remaining sauce, then sprinkle with the cheese. Bake until the enchiladas are hot and the cheese has browned slightly, about 15 minutes.
Garnish the enchiladas with reserved onion rings and cilantro sprigs before serving.