In an oven-safe (no Teflon or plastic) sauté pan over medium-low heat, place onions in one tablespoon olive oil. Add 1/4 teaspoon salt. Allow to cook until onions are softened and transparent, about 5 minutes.
Add the zucchini, mushrooms, and herbs. Allow to cook for an additional 5 to 8 minutes, until the zucchini is softened, all the water released from the vegetables has evaporated, and mushrooms are browned. Add the minced garlic and remaining salt. Cook for 30 seconds longer. Remove from heat.
Add the melting cheese, followed by the Parmesan cheese.
Top with the panko breadcrumbs. Drizzle with remaining olive oil and sprinkle with pepper.
Turn on the broiler. Place the pan under the broiler for 5 to 8 minutes or until the top cheese is golden.
Remove and sprinkle with the parsley before serving, if using. Enjoy!