Melt butter over medium-low heat in a large sauce pan or dutch oven, then add chopped onions and saute until fragrant and translucent.
Add flour a little bit at a time, stirring until well-incorporated. Continue to stir for 2-3 minutes over medium-low heat.
Add water and chicken bouillon paste and stir well (a whisk is even better here).
Remove from heat and add can of green chilies and sour cream, then stir well until incorporated.
Add 1 a cup of your sauce mixture to the bottom of a 9x13 baking dish and set aside.
To your shredded chicken, add the taco seasoning and 1 cup of the sauce mixture. Stir until combined and then divide the chicken mixture into 6 equal parts (I usually just eyeball it).
Add the chicken mixture to each of the 6 tortillas and then fold each end and roll tightly. Line the rolled tortillas up in your baking dish.
Pour the remaining sauce mixture evenly over the tortillas, then top with the cheese.
Bake at 350 degrees until slightly browned and bubbly - about 25 minutes. Remove and let cool for at least 10 minutes.
Top with cilantro, if desired.
