Preheat your oven to 350°F (175°C).

Heat a skillet over medium heat and add 2 tablespoons of olive oil. Once the oil shimmers, toss in the sliced onions and sauté for about 5 minutes, stirring occasionally, until they soften and turn translucent. Add the minced garlic and continue cooking for 2 more minutes until fragrant. Remove the skillet from the heat.

Prepare a baking dish by lightly coating the bottom with olive oil. Begin assembling the casserole by layering the zucchini slices in an even layer at the base. Follow with a layer of sliced tomatoes. Sprinkle the chopped parsley over the tomatoes to add a fresh herbaceous note.

Repeat the layers: another round of zucchini followed by another layer of tomatoes. Scatter the basil chiffonade evenly across this middle layer for added aroma. Complete the casserole with a final layer of zucchini and tomatoes.

Distribute the sautéed onions and garlic evenly across the top layer. Season the dish with a pinch of sea salt and a few generous twists of freshly ground black pepper. Drizzle the remaining olive oil evenly over the top, ensuring the surface glistens. Finally, sprinkle the grated Parmesan cheese across the dish for a savory finish.

Place the baking dish in the preheated oven and bake for 45 to 60 minutes. At the 45-minute mark, the zucchini will still have a bit of bite. For a softer, melt-in-your-mouth texture, continue baking for the full hour.

Once baked, remove the dish from the oven and allow it to rest for about 5 minutes. This short resting time allows the juices to settle. Spoon the flavorful juices from the bottom of the dish over the top before serving for an extra burst of flavor.

Serve warm and enjoy this vibrant, rustic combination of zucchini, tomatoes, and herbs.