Preheat oven to 375°F.
Place a large skillet over medium-high heat and pour in oil. When it's hot, add onions and salt and pepper. Sauté until the onions are softened and beginning to caramelize. Pour into the bottom of a 9" by 13" baking dish.
Begin layering tomato slices along the left side of the pan to form an overlapping row. Overlap zucchini slices along edge of tomato row, and repeat alternating tomatoes and zucchinis, adjusting as necessary until the pan is filled.
Sprinkle with salt and pepper, oregano and grated Parmesan and bake for 30 minutes, or until top is golden and juices are bubbling.
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