Preheat oven to 400 degrees F. Spray a square 2-quart baking dish with olive oil spray.
Slice the unpeeled zucchinis into ½-inch thick slices. Sprinkle with the salt. Place in a colander and allow to drain at room temperature for 30 minutes. This step helps prevent a watery casserole.
Transfer the drained zucchini to a large bowl. Add the marinara sauce, black pepper, dried oregano, garlic powder, and 1 cup mozzarella. Mix everything together.
Transfer the mixture to the prepared baking dish. Sprinkle with the remaining mozzarella and with the Parmesan. Bake uncovered until the cheese is bubbly and browned, about 30 minutes.
Let the casserole rest for 10 minutes before serving. Like all casseroles, this zucchini casserole tastes best when warm or at room temperature.