Preheat oven to 425˚F.
Butter a baking dish and set aside.
Cut the zucchini into ¼-inch slices and sprinkle salt on each slice. Let zucchini slices rest for 15 minutes so the salt can draw out the moisture.
Wipe each of the slices with a paper towel to remove the salt.
Arrange zucchini slices in the baking dish. Set aside.
In a mixing bowl whisk together heavy cream, sour cream, Italian seasoning, onion powder, salt, pepper, and ½ cup parmesan cheese.
Melt butter in a saucepan; stir in garlic and cook for 20 seconds.
To the saucepan, add in the heavy cream mixture; whisk and cook until mixture begins to bubble up. WHISK so that the cream does not start to curdle.
Stir in ½ cup mozzarella; cook for 20 more seconds or until cheese is all melted.
Pour the cream mixture over the zucchini in the baking dish; shake the baking dish to distribute the cream all around the zucchini slices.
Sprinkle the rest of the parmesan and mozzarella cheese over the zucchini.
Bake for 18 to 22 minutes, or until golden brown on top.
Remove from oven and let stand 5 minutes before serving.