Preheat your oven to 400°F. Begin by cooking the chicken and onion with oil in a skillet over medium-high heat until the chicken is fully cooked.

Evenly distribute the cooked chicken among the tortillas and sprinkle 1 1/2 tablespoons of cheese on each.

Roll up the tortillas with the filling inside and place them seam-side down in a 9x13 inch baking dish coated with non-stick spray.

In a saucepan, melt butter over medium heat, mix in flour to create a roux, and cook until it's bubbly. Gradually add chicken broth and bring the mixture to a boil, stirring often.

Remove the saucepan from heat, then stir in the sour cream and green chilies until well combined.

Pour the sour cream sauce over the enchiladas in the baking dish, making sure they are evenly coated.

Sprinkle the remaining cheese on top of the enchiladas and bake in the preheated oven for 20 minutes, or until the cheese is melted and the sauce is bubbly around the edges.