How to Make It
Preheat oven to 350º. In a large saucepan, heat 2 tablespoons of the olive oil. Cook 1 of the onions over moderate heat until soft and translucent, about 5 to 7 minutes. Add the garlic, jalapeño, chili powder, cumin, crushed red pepper, paprika, oregano, sugar, salt and pepper and cook, stirring, for 1 minute. Stir in the tomatoes and remove from heat. Blend in a food processor or blender until smooth. Return the tomato mixture to heat and bring to a simmer. Add the chicken breasts and cook, stirring occasionally, for 20 to 25 minutes, until the chicken is no longer pink and cooked through. Transfer the chicken to a cutting board and use two forks to shred it.
In a large skillet, heat the remaining 2 tablespoons of olive oil over moderate heat. Add the spinach and turn to coat the leaves in oil. Continue to turn until the leaves are wilted, about 3 minutes. Season with salt and pepper.
In a large bowl, combine the chicken, spinach and 1 cup of cheese. Stir in the tomato sauce.
Spread 1 cup of the spinach and chicken mixture on the bottom of a 9-by-13-inch glass baking dish. Arrange 3 eight-inch tortillas or 6 four-inch tortillas evenly on top of the mixture. Spread 1 1/2 cups of the mixture on top of the tortillas and then add another layer of the tortillas. Repeat this, spreading another 1 1/2 cups of the mixture on the tortillas and then adding another layer of the tortillas. Top with the rest of the mixture and then sprinkle on the remaining 2 cups of cheese. Bake for 20 minutes, until the enchiladas warmed through and the cheese is melted.
Serve warm with sour cream, jalapeños, cilantro and minced green onions.