Preheat the oven to 400°F. Spray a 2-quart baking dish with olive oil spray.
Slice the unpeeled zucchini into ½-inch thick slices. Sprinkle the slices with salt. Place them in a colander and allow them 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 of mozzarella. Mix to combine.
Transfer the mixture to the prepared baking dish. Sprinkle it 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 it. Like all casseroles, it tastes best when warm or at room temperature.
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Notes
This is an easy recipe, but one step you should not skip is salting and draining the zucchini. It helps eliminate excess water and keeps the casserole from becoming soggy.
I don't rinse the zucchini after salting and draining it, so there's no need to add salt when seasoning the casserole.
Sometimes, I replace the marinara sauce with the same amount (1 cup) of sour cream. The rest of the recipe remains exactly the same. It creates a different flavor profile - milder and creamier.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Reheat them gently in the microwave at 50% power. They are also surprisingly good when eaten cold! I don't recommend freezing the leftovers.