My Grannie always grew eggplant and we loved it any way she fixed it. I added the Italian sausage when I was grown because I love Italian sausage. But you can leave the sausage out and make it just a veggie dish. My Grannie never added meat to this dish. She made her own spaghetti sauce, too, but I use jarred or canned sauce, and have also used marinara sauce.
Turn on oven broiler. Place eggplant on broiler pan and brush with oil. Broil on both sides untill golden brown. While the eggplant is broiling (watch carefully), fry down the bulk Italian sausage (optional) and drain all grease.
Once eggplant is out of the oven, turn oven on 350 degrees. In an 8x8 or 7x11 inch oven-proof casserole, layer half the eggplant slices then half the sauce. Top with half the sausage and then half the mozzarella cheese. Repeat the layers.
Cover with foil and bake for 45 minutes.
While casserole is baking, make the white sauce by melting the butter in a heavy saucepan. Whisk in flour and continue to whisk over medium heat until mixture is a light tan. Slowly add the milk; whisking constantly. Cook and stir over low heat untill the sauce is smooth and thick. Season lightly with salt and pepper, then remove from heat, stir and cover untill ready to spread over casserole.
After 45 minutes of baking, remove the aluminum foil from casserole dish and spoon white sauce over the top of the casserole. Sprinkle the remaining mozzarella cheese over the top. Turn oven to BROIL and heat heat casserole until top is lightly browned and bubbly. I serve with crisp green salad and garlic bread.