Preheat oven to 375 F. Spray a 2 quart casserole dish with cooking spray and set aside.
Wash zucchini and slice about 1/4 inch thick. If using larger zucchini then slice lengthwise first.
Add olive oil to skillet and preheat over medium high heat. Add zucchini and saute about until crisp-tender, about 5 minutes. If adding fresh garlic then add to skillet to during the last minute of cooking time.
Remove skillet from heat and season zucchini with salt and pepper.
In small bowl, combine the cheddar and Parmesan cheese. Remove one cup and set aside.
In a large bowl, combine the remaining cheese, garlic powder, eggs and sour cream. Fold in zucchini mixture.
Place zucchini mixture in casserole dish and sprinkle with reserved cheese. Bake uncovered, 35-40 minutes or until top is brown and bubbly. Let stand 5 minutes before serving.