Preheat oven to 400F; get out a 9 by 13-inch casserole dish.
Toss the zucchini slices together with the coarse kosher salt in a large bowl. Place the zucchini in a colander and set the colander into the sink (or over a bowl) so the zucchini can sit for 30 minutes and release its excess water.
Meanwhile, add the tomato sauce and tomato paste to a medium saucepan. Bring to a simmer over medium heat, and then turn the heat down slightly and simmer uncovered until thickened, about 8 to 10 minutes, stirring occasionally.
While the tomato sauce is cooking, heat the olive oil in a large skillet over medium heat. Once hot, add the onion and cook until softened, but not browned, about 8 minutes. Add the garlic and cook 1 minute more. Turn off heat and cool slightly.
Add the onion/garlic mixture to a food processor along with 1/2 pack of mozzarella shreds, the parmesan topping, eggs, Worcestershire sauce, and black pepper, and process until well combined, stopping to scrape the sides down as necessary. Add the rosemary and pulse a couple times to combine.
Once the zucchini has drained for 30 minutes, give it a quick rinse under cold water to remove any surface salt, and then pat each slice dry with a paper towel, blotting out as much water as possible.
To layer the lasagna, spread 1/2 cup of tomato sauce onto the bottom of the prepared casserole dish.
Layer 1/2 of the zucchini slices; spread on 1/2 of the creamy onion mixture, top with 1/2 of the remaining tomato sauce, and sprinkle on 1/2 of the remaining mozzarella shreds.
Repeat the previous layer once – top with the remaining 1/2 of the zucchini slices; spread on the remaining 1/2 of the creamy onion mixture, top with the remaining tomato sauce, and sprinkle on the remaining mozzarella shreds.
Cover the dish with foil and bake until warm throughout, about 40 minutes; to crisp the cheese on top a bit, remove the foil during the last 10 minutes of baking.
Let the lasagna stand for 10 minutes before cutting. Serve with more parmesan topping if desired.Recipe by Faith Gorsky, An Edible Mosaic