Preheat the oven to 350°F. Grease a 9x13-inch baking dish and set aside.

Spread the eggplant on a paper towel-lined surface. Sprinkle generously with salt and let sit for 30 minutes. Pat thoroughly dry with paper towels.

In a large skillet, heat the olive oil over medium-high heat. Add the onion and eggplant. Cook for 5 minutes, stirring frequently, until the onion is softened and the eggplant is lightly golden in places. Add the garlic and continue cooking for an additional 30 seconds.

Add the zucchini, red bell pepper, diced tomatoes (with half the liquid drained), dried oregano, salt, and ground black pepper. Reduce the heat to medium-low and simmer, uncovered, for 5 minutes to heat through.

Bring a medium pot of water to a boil and cook the cheese ravioli for 2 minutes less than the package directions. They will finish cooking in the oven. Drain and spread evenly in the prepared baking dish.

Remove the vegetable sauce from the heat and pour evenly over the ravioli. Sprinkle the mozzarella cheese evenly over the top.

Bake, uncovered, for 15 minutes until the cheese is melted.

Let the casserole rest for 5 minutes, then sprinkle with fresh basil and serve hot.