Cook:5 hrs to 6 hrs (low) or 2 1/2 to 3 hours (high)
In a 4- to 5-quart slow cooker, combine chicken, stir-fry vegetables, cheese, and six of the tostada shells, coarsely broken. Pour enchilada sauce over all. Stir to combine.
Cover; cook on low-heat setting for 5 to 6 hours or on low-heat setting for 2 1/2 to 3 hours. Divide mixture among remaining six tostada shells. If desired, top with shredded spinach and dairy sour cream.