Preheat oven to 350 degrees F.
In a 4-quart saucepan over medium heat, add oil. Add the onions, garlic, and salt. Sweat for 3 minutes or until vegetables are translucent and aromatic. Add the remaining ingredients and bring to a simmer. Simmer for 8 hours over low heat, stirring occasionally. Check seasoning.
Cut eggplants in half, scoop out seeds, and sprinkle with salt, cut side up. Place the eggplant halves on a baking sheet and let sit for 1 hour. Place 1 1/2 ounces of sausage in each eggplant half. Place in the oven and bake for 15 minutes. Top each eggplant with some tomato sauce and fresh mozzarella. Place back into oven for 10 minutes or until the cheese melts.