Make the meat sauce. Add 1 tablespoon of unflavored oil in a skillet over low heat. Once hot but not smoking add the ground beef, and season. Cook the meat on medium-low heat until it turns deep brown and absorbs all its liquid. Stir in the onions and garlic. If there is too much fat in the pan, drain the fat before you add the marinara sauce. Stir the marinara sauce, tomato paste, water, and parmesan rind. Mix well until the sauce ingredients are totally blended. Let the sauce simmer until it thickens.
Make the Béchamel (White Sauce). Melt the butter, add the flour and cook for about 5 minutes, stirring constantly, until it resembles a golden paste. Add warm milk and continue stirring with a wire whisk until the sauce begins to thicken. Stir in the salt, pepper, nutmeg, and the shredded cheese. Turn off the heat once the cheese melts away in the white sauce.
Preheat the oven at 375 °F.
Assemble the dish. First add 1/2 the quantity of meat sauce, and then lay over the zucchini slices on top. Top the zucchini with the other half of the meat sauce. Level the meat sauce out and then spread the white sauce all over the casserole. Even out the white sauce with an offset spatula or the back of a spoon. Sprinkle grated parmesan cheese on top of the white sauce.
Bake the casserole. Cover the casserole with parchment paper first and then aluminum foil and bake the casserole in a preheated oven at 375 °F for 30-40 minutes until the bechamel sauce is golden brown, the zucchini is soft and fully cooked. Remove the parchment and the aluminum and let the casserole bubble under the broil for 5 minutes, to get an irresistible golden color.
Serve Koussa Bel Bechamel. Preferably, let the Koussa Bel Bechamel rest 15-20 minutes after it leaves the oven so the layers can slightly consolidate, which facilitates dishing out. Serve it with Salata Baladi, Egyptian green salad as they beautifully go well together.
Store. The assembled, unbaked Koussa Bel Bechamel can be wrapped (in its baking dish) in plastic wrap and then in aluminum foil, and frozen for up to 1 month. Defrost at room temperature or overnight in the refrigerator. Remember to remove the plastic wrap and re-cover with parchment paper and foil before baking.