Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with olive oil.
Place the sliced zucchini in a colander, sprinkle with kosher salt, and let it rest for 20 minutes. Pat the zucchini dry thoroughly with paper towels.
In a large bowl, combine the softened cream cheese, sour cream, minced garlic, Italian seasoning, onion powder, black pepper, 1 cup mozzarella, and ½ cup cheddar. Mix until smooth and creamy.
Arrange half of the zucchini in the prepared baking dish.
Spread half of the cheese mixture over the zucchini.
Repeat with the remaining zucchini and cheese mixture.
Sprinkle the remaining mozzarella, cheddar, and Parmesan evenly over the top.
Bake uncovered for 30–35 minutes, or until the zucchini is tender and the cheese is melted and bubbling.
Broil for 2–3 minutes until the top is golden brown.
Let the casserole rest for 10 minutes before serving.
Garnish with chopped fresh parsley if desired and serve warm.