These are a really nice, quick meal when you don't really feel like "cooking." Also, if you're trying to cut down on the amount of meat you're eating, this is a nice recipe. Portobellos are very "beefy" and filling.
My changes:
Omitted the butter- I sprayed the outside parts of the tortillas with Pam Olive Oil spray to give them a little crunch after being baked in the oven.
Used 2% sharp shredded cheddar
Used whole-grain tortillas