Heat a pot over medium heat. Swirl canola oil in bottom of pan. Add onions, jalapeños, and garlic. Cook until fragrant and tender.
Add spices, sugar, and salt. Stir and allow spices to toast, 2 minutes.
Pour in tomato sauce and water. Simmer 5 minutes.
[At this point you can cool sauce, blend sauce in blender, and cover and store in refrigerator until ready to use or move on with making enchiladas]
To Make Chicken Enchiladas:
Place 4 chicken breast halves in pot with sauce and simmer, 1 hour, until chicken shreds easily with a fork and has absorbed all the flavor of the sauce.
Preheat oven to 425. Grease a 13x9 pan.
Remove pot from heat and transfer chicken to cutting board. Shred chicken with forks.
Transfer shredded chicken to bowl, season with salt. Mix and set aside.
Transfer cooled enchilada sauce to blender and whirl until smooth. Put 1/4 cup sauce into bowl of shredded chicken. Add 1/2c shredded cheese to bowl with chicken and sauce. Stir to combine.
Steam corn tortillas in a tortilla warmer in the microwave, about 2 minutes, until hot and pliable. *If you do not have a tortilla warmer, a ziploc bag works well.
Spoon shredded chicken mixture (about 1/4 cup) onto center of warm tortilla, roll, and place seam-side down in greased pan. Repeat until all the chicken has been used.
Pour enchilada sauce over the entire pan of enchiladas.
Sprinkle remaining cheese evenly over top. Sprinkle green onions over cheese.
Bake 20 minutes more, or until sauce is bubbly and cheese is gooey and melted.