Spread the zucchini slices in a single layer over colanders and/or wire cooling racks. Sprinkle lightly with salt (about 1/2 teaspoon per pound), and let stand for 30 minutes. Rinse and gently pat dry, trying to remove as much excess water as possible. (see notes)
Preheat the oven to 350 degrees F. Line 2 baking sheets with foil or parchment and spray lightly with cooking spray.
Place the zucchini in a single layer. Roast for 10-15 minutes, then remove from the oven and transfer to paper towel-lined plates or cutting boards to drain away any excess moisture while preparing the other ingredients. Also blot the tops dry as needed.
In Dutch oven or heavy-bottom saucepan set over medium-low heat, brown the ground beef for 5-7 minutes. Drain the fat through a colander (don't pour the fat down the drain; discard it).
In the same pan, heat the olive oil. Cook the onions until soft and lightly browned around the edges, 3-5 minutes. Add the garlic and thyme, then cook for another 1-2 minutes. Return the meat to the pan along with the marinara sauce, and stir to combine the ingredients. Remove from the heat and set aside.
In a large bowl, stir together the ricotta, eggs, 3/4 cup parmesan, 1 cup mozzarella, oregano, 1/2 teaspoon of salt and 1/4 teaspoon pepper.
Grease a 9x13 inch casserole dish (or similar size) using olive oil or cooking spray. Spread 2 cups of meat sauce on the bottom. Top with approximately 10 slices of zucchini. Top with half the cheese filling, spreading evenly. Top with 1 cup sauce, a layer of zucchini, the remaining cheese, a layer of zucchini, and the remaining sauce. Sprinkle the remaining parmesan and mozzarella evenly over the top. Bake until the cheese is bubbly and the edges are darkening up, approximately 50 minutes. Allow to set for 20 minutes before slicing. Serve warm.
This zucchini lasagna will keep in the refrigerator, covered, for up to 5 days. If you plan to keep leftovers, I recommend slicing portions within 3 days, and storing in individual freezer containers or individually wrapped in a large freezer bag.
Video
Notes
I used a mandoline to slice the zucchini quickly and evenly. The 3/16th setting is a good option.The first time I prepared this, I used 5 zucchini and 12 slices of zucchini per layer (see photos), which still led to a small amount of water pooling on the plate. The second time, I used 4 zucchini and 10 slices. It eliminated the water issue, and you can’t tell the difference in the amount of zucchini.Don’t be tempted to skip the zucchini salting and roasting; you want to remove as much liquid as possible from the zucchini to keep it from seeping into the lasagna. Excess liquid will pool onto your plate after slicing (which can be dabbed away with a paper towel, but it’s nicer to avoid it).Since this recipe already has several steps, I like to save time by using store-bought marinara sauce. It’s slightly pricer than other brands, but I love Rao’s Homemade Marinara Sauce. It actually tastes homemade. You can use your favorite brand. You could also use my homemade meat sauce.The lasagna can be assembled the day beforehand and then covered and refrigerated until ready to bake. You may need to increase the bake time slightly, since the ingredients will be chilled.