In a large skillet, heat the oil & add the onion , cooking until softened, about 5 minutes . Set aside.
Pour about 1/2C enchilada sauce into the cooker, and spread around.
In layers add 1/4 of the tortilla strips, 1/4 of remaining sauce, 1/3 of the onions, 1/3 of the chicken, and 1/4 of the cheese. Repeat the layers two more times ending with the cheese. Finish the casserole with the remaining tortilla strips, sauce, & cheese.
Spoon the sour cream over the surface of the casserole in big dollops. Use a spatula or the back of a large spoon to gently spread it all around without disturbing the layers. Cover and cook on high for 2 hours, or on low 4-5 hours.
To serve, use a long handled spoon to reach down through all the layers for each serving. Make sure each diner gets some of the sour cream.