Cut and remove the ends from all of the zucchini. Then, cut each one lengthwise into 4-5 slices about ¼”-thick. Lay the zucchini flat on the cutting board and sprinkle with ½ teaspoon of salt. Let sit for 20-30 minutes. Dab the excess liquid with a paper towel every 5-10 minutes to remove it.
In a large skillet add the oil and onion. Sauté over medium heat for 2-3 minutes, or until the onion becomes tender. Add the garlic and continue sautéing for an additional 30 seconds.
Push the onion and garlic to the side of the skillet and add the ground beef. Cook for 7-8 minutes, or until the meat is cooked through and no longer pink. Drain off any excess liquid, if needed. Using a potato masher, mash the meat until it forms fine crumbles.
Pour in the marinara sauce along with the nutmeg, ¾ teaspoon of salt, and black pepper. Stir together, and then bring the sauce to a boil. Reduce the heat to medium-low and simmer for 10 minutes, stirring occasionally.
While the sauce is simmering, mix together the ricotta cheese, egg, Italian seasoning, Parmesan cheese, and ¼ teaspoon salt in a medium-sized bowl.
Spray a large 9x13 inch baking dish with non-stick cooking spray. Pour in a few spoonfuls of the meat sauce and then layer half of the zucchini slices on top. It’s ok if a few of them overlap.
Spread the ricotta cheese mixture over the slices and then top with half of the meat sauce. Repeat the layers: zucchini slices, ricotta cheese, ending with the meat sauce. Sprinkle the mozzarella cheese on top.
Bake at 375 degrees for 35-40 minutes, uncovered, or until the zucchini is tender and the cheese is lightly golden.
Let sit for at least 10-15 minutes before serving with a sprinkle of fresh parsley or basil and enjoy!