Preheat oven to 375° F. Coat a 9 x 13-inch baking dish with cooking spray, sprinkle in garlic and arrange half of the potatoes in the dish. Evenly add 1/2 the cheddar.
In a small bowl, mix the nonfat mayonnaise, evaporated milk, white pepper and thyme. Spoon half of this mixture over cheese. Cover with the sliced zucchini. Sprinkle on remaining cheddar cheese, and spoon remaining milk mixture on top of cheese.
Arrange remaining potato slices on top and sprinkle Parmesan over all. Cover tightly with foil and bake 50 minutes.
Uncover and broil 6 inches from heat source 4 - 6 minutes or until golden. Serve hot or at room temperature.