Position a rack in the bottom third of the oven and preheat the oven to 350 degrees F. Lightly coat a 13 by 9-inch pan and a piece of foil large enough to cover the pan with nonstick cooking spray.
To make the filling: Heat the oil in a 12-inch nonstick skillet over medium-high heat. Add the onion and cook, stirring frequently, until it is clear, about 4 minutes. Add the chicken, chiles and chili powder, and cook for 2 minutes, stirring constantly. Remove from the heat and stir in the corn, spinach, sour cream, cheese, and salt.
Place a stack of 6 tortillas between damp paper towels and microwave on high power until the tortillas are hot and soft enough to roll, about 1 minute. Cover with a cloth towel to keep warm. Remove 1 tortilla at a time, keeping the other tortillas covered, and place a rounded 1/3 cup of the filling down to the center. Roll into a cylinder and place in the prepared pan, seam side down and close enough to touch. Repeat with the remaining tortillas. Cover the pan tightly with foil and bake until the enchiladas are hot, about 30 minutes.
To make the sauce: Meanwhile, stir the taco sauce and sour cream together in a small bowl. Uncover the pan and spread the sauce over the hot enchiladas. Sprinkle cheese and cilantro over the top. Return to the oven, uncovered, and bake until the cheese melts and the sauce is bubbly, about 10 minutes.
Recipe courtesy Carla Fitzgerald Williams, Rotisserie Chickens to the Rescue!, Hyperion Books, 2003