Prepare the base for the moussaka. First, slice the eggplants and season them generously with salt. Set the eggplant slices aside for about 15 minutes. Then rinse the eggplants with water and pat them dry with paper towels. After that, spray a skillet with some cooking oil, and over medium heat, fry the eggplant slices until brown on each side.
Prepare the meat sauce. In a large pan, fry the beef mince with chopped onion over medium heat until fully cooked (for about 10-15 minutes). Add the minced garlic, marinara sauce, salt, and pepper. Cook for a few more minutes and then set aside.
Prepare the white sauce. In a saucepan, mix cream cheese, heavy cream, and water. Cook it, constantly stirring, over low heat. After the cream cheese has melted, temper the egg yolk and slowly add it into the white sauce. Continue stirring until the sauce thickens, but be careful not to let it boil. Add some salt and set aside.
Assemble the moussaka. Spray the bottom of the baking dish with cooking oil. Layer half of the eggplant slices on the bottom of the baking dish, evenly spread out the meat sauce, and place the second layer of the eggplant slices on top of it. Then, pour the white sauce on top of the eggplant slices.
Preheat the oven to 350 F and bake the moussaka for about 35-40 minutes, just until it acquires a light crust. Let the moussaka cool down a bit before serving.