Preheat your oven to 350°F. Heat 1 tablespoon olive oil in a small skillet over medium heat. Add the chopped onion and sauté until soft and translucent, about 3–4 minutes. Stir in the garlic and cook for 30 seconds, until fragrant.

Spread about one-third of the onion and garlic mixture over the bottom of a medium round tart dish or casserole dish. Layer the zucchini and tomato slices in a circle, alternating 1 slice zucchini and 1 slice tomato, until the dish is filled.

Spoon the remaining onion and garlic mixture evenly over the top. Drizzle lightly with the remaining olive oil, then season with salt, black pepper, and oregano. Sprinkle the Parmesan evenly over everything.

Bake for about 35 minutes, or until the vegetables are tender and the top is lightly golden. Let cool for a few minutes before serving.