Ladle 1 cup enchilada sauce into large slow cooker, or Instant Pot. Layer 2-1/2-3 tortillas over the sauce, tearing the last one to two tortillas so the pieces cover all the sauce and fit the shape of the cooking dish.
Top with 1/3 of frozen spinach and 1/3 of chicken, sprinkled evenly over the tortillas. Top the spinach and chicken with 1/3 of cheese filling, spreading it evenly. (It will be a generous amount.)
Ladle 1 cup enchilada sauce over the cheese filling and top again with 3 tortillas.
Continue this assembly until all ingredients have been used, finishing with final layer of enchilada sauce.
Top all with shredded cheddar cheese.
If using Crock Pot, place dish into base and set timer for 3.5 hours, on low heat. (Note: Pressure cookers vary in how hot they run; so cooking times will vary.)
If using Instant Pot, make sure rubber ring is in place. Seal lid and shut steam valve. Set "Manual" button to 55 minutes. When timer goes off, do a Natural Release of pressure for 30 minutes; then press "Cancel" button and do a Quick Release. It will be saucy, but use a butter knife to cut down and through tortillas (cut in sections like cutting a pie). I then use a ladle in one hand and large serving spoon in the other so I scoop big saucy sections onto each plate or into broad bowls.
Also serve Crock Pot version of finished casserole with broad serving spoon for scooping up the sauciness. Garnish each with desired condiments: grated cheese, fresh cilantro, sliced olives, shredded lettuce, pico de gallo salsa and/or sour cream.