Adjust an oven rack to the middle position and heat the oven to 400F.

Heat olive oil over medium low heat. Saute onion and jalapeno in oil until the onion is translucent. Stir in garlic, chili power, 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.

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 a meat 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.

Once the chicken is cool enough to handle, shred into bite-sized pieces. Add to the bowl along with the onion mixture. Add in ¼ cup of your tomato sauce sauce, 1 cup cheddar cheese, and the cilantro. Toss to combine.

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 ⅓ 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. Pour remaining sauce over the enchiladas. 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.

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.