Source: Tide and Thyme, who adapted it from America's Test Kitchen
1 medium onion , chopped fine
1 tsp. vegetable oil
3 medium cloves garlic , minced
3 tbsp. chili powder
3 tsp. ground cumin
2 teaspoons sugar
2 cans (8 ounces each) tomato sauce
1 cup water
1 pound boneless, skinless chicken breasts (about 2 large breasts)
8 ounces cheddar cheese, shredded
3 tbsp. pickled jalapenos, chopped
1/2 cup minced fresh cilantro
12 (6-inch) soft corn tortillas
Vegetable cooking spray
Ground black pepper
1. Adjust an oven rack to the middle position and heat the oven to 425 degrees. Combine the onion, oil, and 1/2 teaspoon salt in a large saucepan. Cover and cook over medium-low heat, stirring often, until the onions have softened, 8 to 10 minutes. Stir in the garlic, chili powder, cumin, and sugar, and cook until fragrant, about 30 seconds. Stir in the tomato sauce and water, bring to a simmer, and cook until slightly thickened, about 5 minutes.
2. Nestle the chicken into the sauce. Reduce the heat to low, cover, and cook until the chicken is no longer pink in the center, and the thickest part registers 160 degrees on an instant-read thermometer, 10 to 12 minutes. Transfer the chicken to a plate; set aside to cool. Strain the sauce through a medium-mesh strainer into a medium bowl, pressing on the onions to extract as much liquid as possible. Place onion mixture in a large bowl and set aside. Season the sauce with salt and pepper to taste.
3. Once the chicken is cool enough to handle, shred into bite-sized pieces. Add to the bowl along with the onion mixture. Add in 1/4 cup enchilada sauce, 1 cup cheddar cheese, the chopped jalapeños, and the cilantro. Toss to combine.
4. Stack the tortillas on a microwave-safe plate, cover with plastic wrap, and microwave on high until warm and pliable, 40 to 60 seconds. Spread the warm tortillas out over a clean work surface. Place 1/3 cup of the chicken mixture evenly down the center of each tortilla. Tightly roll each tortilla around the filling and lay them seam-side down in a 13 by 9-inch baking dish.
5. Lightly spray the tops of the enchiladas with vegetable oil spray. Place in the oven, uncovered, for about 7 minutes until the tortillas are starting to slightly brown on the top. (If you dont’ like a bit of crunch in your enchiladas, just skip this step completely)
6. Reduce oven temperature to 400. Remove enchiladas from oven and pour remaining sauce over to coat them thoroughly. Sprinkle the remaining 1 cup cheddar down the center of the enchiladas. Cover the baking dish with foil and bake until the enchiladas are heated through, 20 to 25 minutes.
7. Remove the foil and continue to bake until the cheese browns, about 5 minutes longer. Remove from oven and let stand for 10 minutes before serving.