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Combine the onion, jalapeno and oil in a large saucepan over medium heat. Cook, stirring often, until the onions and peppers have softened, about 10 minutes.
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Stir in the garlic, chili powder, cumin, salt and sugar, and cook just until fragrant, less than 30 seconds.
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Mix in the tomato sauce and water and bring to a simmer.
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Lower the heat and cook until slightly thickened, about 5 minutes.
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Nestle the chicken into the sauce. Reduce the heat to low, cover and cook until the chicken is completely cooked through (160˚ F on an instant-read thermometer), about 20 minutes.
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Transfer the chicken to a plate and set aside to cool.
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Strain the sauce through a large mesh strainer into a medium bowl. Transfer the reserved solids to a large bowl and set aside. Season the sauce with additional salt and pepper to taste.
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Shred the chicken into bite size pieces and add to the bowl with the onion mixture. Add in ¼ cup of the sauce, 1 cup of shredded cheese, and the cilantro. Stir to combine.
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Preheat the oven to 275˚ F. Oil a 9 x 13" baking dish with cooking spray.
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Wrap your stack of tortillas in a damp towel/paper towel and microwave for 30-45 seconds, until warm and pliable.
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Spoon 1/3 cup of the chicken mixture evenly down the center of a tortilla. Roll up the tortilla around the filling tightly, and place in the prepared baking dish, seam-side down.
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Repeat with the remaining filling and tortillas.
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Bake for 25 to 30 minutes or until warmed through and cheese is bubbly and top starting to brown.
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Garnish with additional cilantro and sliced avocado and serve!