Store any leftover Mexican chicken casserole in an airtight container in the fridge for up to 4 days. When ready to reheat, cut individual servings and transfer to a microwave-safe plate. Tent with parchment paper and microwave for 1 minute 45 seconds on high power. (Microwave cooking times vary depending on your machine.) Or you can reheat in the oven, by covering the casserole dish with parchment paper and foil and bake until the center of the casserole is hot and warmed through.