Preheat oven to 350 degrees F. Spray a 13 x 9 baking pan with non-stick cooking spray.: As the oven warms, you will notice a slight dry warmth in the kitchen, a sign the environment is ready to transform the assembled casserole. This step ensures even baking and prevents sticking, helping the bottom layer of tortillas to crisp slightly at the edges while staying tender in the center. If your pan is not well greased, the bottom can cling and tear when serving, so make sure to coat corners and sides. A metal pan heats more quickly than a glass pan, so watch for color differences when it bakes.
In a medium size bowl, combine sour cream and taco seasoning and mix well. Chop or shred chicken into small pieces. Add to sour cream mixture and stir to combine.: The first sensory cue is the spicy scent of the taco seasoning blooming into the cool tang of the sour cream , creating a fragrant, creamy base. Mixing the chicken in helps the flavors marry so each bite is seasoned, not just the surface. This technique ensures even distribution rather than pockets of blandness. A frequent mistake is using very large chicken pieces, which prevents uniform flavor in every forkful, so shred or chop to small, consistent sizes.
Pour approximately 1/2 inch of enchilada sauce into the baking dish, making sure to fully cover the bottom of the dish.: When you pour the enchilada sauce you should see a glossy red or green sheen settle across the base, which protects the bottom tortillas from drying and creates a saucy foundation. This coating also helps distribute heat so the bottom layer cooks evenly. Pouring too little can cause sticking and dryness, while too much may make the casserole soupy. Aim for a thin, even coat to strike the right balance.
Place a layer of tortillas onto the enchilada sauce to cover the bottom of the pan. This will be approximately 6 enchilada pieces.: The visual cue is rectangles overlapping slightly, forming a mostly covered surface. The tortillas should be somewhat saucy but not saturated; they will soften during baking and bind the layers. Proper placement avoids gaps that can lead to uneven structure. If you crowd them or leave big holes, the filling can bubble up and make slices fragile when serving.
Top with 1/2 the chicken and sour cream mixture, then sprinkle 1/3 of the shredded cheese on top, followed by 1/3 of the remaining enchilada sauce. Repeat for a second layer.: At this moment you will notice the creaminess of the sour cream mix contrasting with the shredded chicken . The cheese adds a salty, fatty note that melts into a cohesive blanket, while the sauce ties everything together. Layering in this order builds balanced bites that are not overloaded with sauce or dry spots. A common misstep is piling too much filling in one layer which makes cutting neat portions difficult, so measure roughly as you go.
Dip or coat the remaining tortillas with enchilada sauce, then place a final layer on top. If there is any enchilada sauce remaining, drizzle over top. Top with remaining cheese.: When you coat the top tortillas , listen for the soft slosh of sauce as it clings, and see the sauce sheen that will help the final cheese melt into a golden finish. This top layer seals moisture inside and creates a pleasing contrasting texture between the creamy interior and the browned topping. Avoid drowning the top in sauce which can prevent a nicely browned cheese crust; aim for an even light drizzle.
Cover baking pan with foil and bake for 45 minutes. Allow to cool for approximately five minutes prior to serving. If desired, garnish with tomatoes, green onions, black olives and additional sour cream.: While baking the kitchen will fill with the savory perfume of enchilada sauce and melting cheese . The foil traps steam and helps the layers meld without drying. Letting it rest for about five minutes after baking allows the layers to set so slices hold together cleanly. A typical error is cutting immediately which leads to sloppy portions; be patient and let it rest. If you over bake, the edges may crisp too much, so check for bubbling and golden cheese rather than relying only on the timer.