I cut two zucchini in half, both across and long ways. Then I sliced them with the mandolin fairly thin and placed the first layer in my greased 8x8 pan. Since it's just myself and my husband, this was the perfect size dish for us. But since it ended up so tasty, I'm planning on making it in the full-size pan next time.
And voila! This really was tasty AND low calorie AND gluten-free. You could also make it dairy free by removing the cheese or using dairy-free cheese. I'm planning to experiment with this more . . . maybe substituting shredded chicken for the beef. Or making an enchilada-esque lasagna.