Place the eggplant slice in a single layer on paper towels, sprinkle lightly with salt, and set aside for 30 minutes to draw out the moisture. Heat a skillet over high heat, add some of the olive oil and fry a batch of eggplant slices on both sides until lightly browned. Place the slices on fresh paper towels to soak up the oil. Repeat with remaining eggplant slices.
Meanwhile, bring a pot of salted water to a boil and add the potato slices. Boil for 5 minutes, drain, and rinse with cold water to prevent further cooking.
In a large skillet over medium heat, heat the olive oil and brown the ground beef and sausage until no pink remains. Add the onions and garlic and continue to cook until the onions are soft, about 5-7 minutes. Add the crushed tomatoes, tomato paste, all the seasonings, the bay leaf, and the wine. Stir to combine. Bring to a boil, reduce the heat to low, cover and simmer for 30 minutes. Remove the lid and allow it to simmer for another 10 minutes, or almost all of the liquid is absorbed. This is important or the casserole will be too watery. Allow the sauce to cool, and then stir in beaten egg.
To make the bechamel sauce, melt the butter over medium heat. Add the flour and use a wire whisk to blend it with the butter. Continue whisking for another 1-2 minutes. Gradually add the milk while continually whisking to prevent lumps. Whisk until the mixture is smooth. Add the Parmesan cheese, salt and nutmeg. Continue whisking until all clumps have dissolved (if you haven't caught on to the trend yet, this sauce requires a lot of whisking!). Continue to whisk (there we go again!) the sauce until it thickens and has simmered for about 3 minutes. Turn off heat and remove the saucepan from the burner. Allow the sauce to cool for about 20 minutes. Then whisk in the egg yolks until thoroughly blended.
Grease a 9x13 inch baking dish. Sprinkle a thin, even layer of fine bread crumbs on the bottom of the dish (this will help absorb the liquid from the vegetables). Arrange a layer of slightly overlapping potatoes on the bottom of the baking dish. Next add a layer of eggplant slices, slightly overlapping them. Evenly spread out the meat sauce on top. Add another layer of eggplant slices and even top with the béchamel sauce.
Preheat the oven to 350 degrees F and bake, uncovered, for 1 hour.
Remove the moussaka and allow it to sit for 15 minutes before serving to allow the liquid in the bottom of the dish to absorb.