Preheat your oven to 375°F (190°C).
Prepare the eggplant by slicing it into 1/4-inch rounds and salting them in a colander.
Rinse the salted eggplant slices under cold water and pat dry with paper towels.
In a large skillet, heat olive oil over medium heat. Add the onions and sauté until translucent, about 5 minutes.
Add the garlic and Italian sausage, breaking it apart as it browns.
Once browned, stir in the crushed tomatoes, oregano, salt, and pepper. Let simmer for 10 minutes.
In a large greased baking dish, layer half of the eggplant slices at the bottom.
Spread half of the sausage mixture over the eggplant, followed by a layer of mozzarella and Parmesan.
Repeat with the remaining eggplant, sausage mixture, and cheeses.
Cover the baking dish with foil and bake for 30 minutes, then remove the foil and bake for an additional 15-20 minutes until golden and bubbly.
Let the gratin cool for 10 minutes before serving.