Heat the oil in a skillet over medium high heat. Add the onions and cook until softened about 5 minutes.
Divide the tortillas and cheese into 4 piles and the onions, beans and chicken into 3 piles. (This makes it easy to make the layers.)
Pour ½ cup of enchilada sauce over the bottom of the slow cooker insert. Tilt to spread it around.
Place a layer of tortillas in the insert. Add ¼ of the rest of the sauce. Then add one pile of the onions, beans and chicken and top with the cheese. Repeat these layers two more times ending with a cheese layer.
Finish the layers with a final tortilla strips, sauce and cheese layer.
Spoon the sour cream over the top.
Cook on high for 2 hours or low for 4-5 hours.
Video
Notes
Putting the ingredients in small piles helped make the layering go easy for me. You can really do it anyway you want, it comes out good no matter how you layer it!