In a large saucepan, combine the tomatoes, onion, butter, and salt. Bring to a boil, then reduce heat and let simmer for about 45 minutes. Break up any large tomato chunks with a spoon, remove onion, and set aside. (This can be done a day ahead to save time.)

In a medium bowl, whisk eggs, pesto and milk. In a second bowl, toss bread crumbs and 1/4 cup of Parmesan.

Set aside 1/2 cup of the sauce. Coat slow cooker bowl with nonstick cooking spray. Spoon 1/2 cup of the sauce on bottom of slow cooker.

Dip each eggplant slice in egg mixture, then bread crumb mixture, and place on a cutting board. Layer one-third of the eggplant slices into slow cooker (about 8 pieces). Top with 1 cup sauce and 2/3 cup Italian cheese blend. Repeat twice. Cover and cook on LOW for 5 1/2 to 6 hours.

During last 30 minutes of cooking time, bring a large pot of lightly salted water to a boil. Add spaghetti and cook 10 minutes. Drain and toss with remaining 2 tablespoons Parmesan and the olive oil

Heat reserved sauce gently in microwave or saucepan. Serve alongside eggplant and spaghetti. Garnish with basil, if using.