Saute aromatics. In a skillet over medium heat, melt butter, sauté diced yellow onion until translucent. Then add minced garlic and sauté for another minute.
Pour in the heavy cream and add sea salt, black pepper, dry thyme, and dry oregano. Bring the mixture to a boil.
Add Parmesan. Stir in the shredded Parmesan cheese and mix until fully incorporated. Remove from heat.
Prep zucchini. Trim the ends of the zucchinis and slice them into rounds.
Transfer to dish. Arrange the zucchini slices side by side in a round baking dish; the one used here has a 9-inch diameter.
Top with sauce. Pour the prepared sauce over the zucchini, spreading it evenly to cover the slices.
Bake. Place the mozzarella cheese pieces on top and bake in a preheated oven at 400°F for 40-45 minutes. Check that the zucchini is fork-tender before removing the casserole from the oven.
