Preheat the oven to 350ºF and line 2 large baking sheets with parchment paper. Cut the ends off the zucchini, then use a mandolin slicer or sharp knife to cut them into thin strips, about a 1/8-inch thick. You should get about 14-15 slices from each large zucchini; about 30 in total. (Or use 3 medium zucchini to get 30 slices.)
Arrange the zucchini slices into a single layer on the prepared baking sheets and sprinkle them lightly with salt. Place the pans in the oven to cook for 10 minutes. This will help to remove excess moisture from the zucchini, so your lasagna won't turn out too watery. When the timer goes off, set them aside to cool briefly.
While the zucchini is in the oven, add the olive oil to a skillet over medium-high heat and saute the garlic for 1 minute, just until it smells fragrant. Add in the ground beef and a 1/2 teaspoon of salt, and stir, breaking up the meat as you go. Cook until the beef is browned, and no longer pink, about 5 to 8 minutes.
Drain any grease from the pan with the ground beef. Set aside a ½ cup of marinara sauce to use on the bottom of the lasagna pan, then add the rest of the sauce into the ground beef. (There should be roughly 1 ¼ cups of sauce to add in.) Set it aside until you're ready to assemble.
Next, add the ricotta to a large mixing bowl, and add in the egg, oregano, and a ¼ teaspoon of salt. Mix well and set it aside.
To assemble the lasagna, spread the reserved ½ cup of marinara sauce over the bottom of a 9-by-13-inch baking dish. Add a layer of zucchini noodles across the bottom of the pan, using about 10 slices. It's okay if they don't overlap. Add half of the ricotta mixture to the top of the zucchini noodles, and gently spread it out. Then top that with half of the meat sauce, spreading it out. Top with 1 cup of shredded mozzarella cheese.
Repeat the layers, starting with another layer of zucchini, followed by the rest of the ricotta, meat sauce, and another cup of mozzarella cheese. Finish the lasagna off with a final layer of zucchini noodles, and sprinkle a little more mozarella cheese on top, about a ½ cup.
Bake the zucchini lasagna for 30 minutes at 350ºF, or until the center of the lasagna reaches 160ºF, to make sure the egg is cooked through. For a golden top, turn on the oven's broiler and let the cheese bubble for 2 to 3 minutes. Watch closely, so the top doesn't burn. Then remove the lasagna from the oven and let it cool for at least 10 minutes before slicing and serving. It's normal for a little bit of liquid to settle at the bottom of the pan as the lasagna rests, FYI. Leftover lasagna can be stored tightly covered in the fridge for up to 4 days.
Nutrition information is for 1 of 8 slices. This information is automatically calculated, and is just an estimate, not a guarantee.
This cook time assumes you will cook the ground beef during the same 10 minutes that zucchini noodles are cooking in the oven. If you don't multitask, the timing might be slightly longer.
For a vegetarian lasagna, replace the ground beef with 2 (15 ounce) cans of lentils that have been drained and rinsed.
For a low-dairy option, try using vegan ricotta cheese instead. You can also use the fillings from my vegan lasagna, for a totally dairy-free recipe.