Prep the zucchini: Lay the zucchini slices on paper towels in a single layer. Sprinkle salt over the top and allow them to sit for about 10 minutes to release moisture. Pat them dry with fresh paper towels to remove excess water.
Prepare the white sauce: Stir the gelatin into water in a small bowl and let it sit for 5 minutes. In a saucepan over low heat, warm the cream cheese and heavy cream, stirring until smooth. Mix the gelatin mixture into the melted cheese until fully incorporated. In a separate bowl, whisk the egg yolk lightly. Gradually blend about ¼ cup of the warm sauce into the yolk, then pour this back into the saucepan. Continue stirring constantly for around 2 minutes until slightly thickened. Set aside to cool and allow the sauce to thicken further.
Make the meat sauce: In a large skillet over medium-high heat, cook the ground beef, breaking it apart as it browns for about 5 minutes. Add the beef stock, vinegar, oregano, basil, rosemary, thyme, and salt. Stir and bring to a simmer, reducing the heat to low. Cook for about 5 more minutes until most of the liquid evaporates.
Mix the ricotta filling: In a bowl, combine the eggs, ricotta cheese, and parsley. Stir until smooth.
Assemble the lasagna: Grease an 8-inch square baking dish with softened butter. Cover the bottom with about one-third of the zucchini slices, overlapping as needed. Spread half of the meat sauce on top, then layer with half of the ricotta mixture, half of the white sauce, and half of the mozzarella cheese. Repeat the layers once more. Finish by topping with the remaining zucchini slices. If desired, sprinkle Parmesan cheese over the top.
Bake: Cover the dish with foil and bake at 325°F for 45 minutes. Remove the foil, increase the oven temperature to 375°F, and bake for another 15 to 20 minutes until the top is browned and the zucchini is tender.
Serve: Let the lasagna rest for 5 minutes before slicing and serving. Store leftovers covered in the refrigerator for up to 4 days.