Preheat the oven to 350 degrees. Spray a 9×13-inch baking dish with non-stick cooking spray.
In a large bowl, combine the shredded chicken, half of the grated cheese, half of the tomatoes with green chilies, and half of the taco seasoning.
In a medium pan over medium heat, melt the butter. Whisk in the flour and cook for 2 minutes, stirring constantly.
Slowly pour in the chicken stock, ½ cup at a time, stirring continuously to avoid breaking the roux. Continue until the sauce is well combined and thickened.
Reduce heat to low and add the sour cream, the remaining half of the tomatoes and green chilies, taco seasoning, and cheese. Stir until the cheese is melted and the sauce is smooth.
Microwave the corn tortillas, covered with a damp towel, for 1 minute. Spoon 2 tablespoons of the chicken mixture into the center of each tortilla, roll them up, and place them seam side down in the prepared baking dish.
Once all the tortillas are rolled, pour the sauce over them, ensuring they are well-covered. Optionally, add jalapenos over the top for an extra layer of flavor.
Bake in the preheated oven for 20-25 minutes or until the enchiladas are bubbly on all sides.
Optional: Let the enchiladas cool for 10 minutes before serving. Dig in and enjoy your delicious sour cream chicken enchiladas!