In a medium saucepan, brown the ground beef and season with salt. Drain excess fat.
Add olive oil to the pan and sauté garlic and onions for about 2 minutes. Return the meat to the pan, then add crushed tomatoes, basil, salt, and black pepper. Simmer on low for 30-40 minutes covered. Do not add extra water.
Slice zucchini into 1/8 inch thick slices, sprinkle lightly with salt and set aside for 10 minutes. Blot excess moisture with a paper towel.
Preheat a grill or grill pan to medium-high and grill zucchini slices 2 to 3 minutes per side until slightly browned. Drain on paper towels.
Preheat oven to 375°F.
In a medium bowl, mix ricotta cheese, Parmigiano Reggiano, and egg until well combined.
In a 9x12 casserole dish, spread 1/2 cup sauce on the bottom. Layer grilled zucchini, spread 1/2 cup ricotta mixture, and sprinkle 1 cup mozzarella cheese. Repeat layers until ingredients are used, ending with zucchini and sauce on top. Cover with foil and bake 30 minutes.
Uncover foil and bake 20 minutes to thicken sauce. Add remaining 1 cup mozzarella cheese on top and bake an additional 10 minutes until melted.
Let the lasagna stand for 5 to 10 minutes before serving.
Notes
This zucchini lasagna is perfect for a low-carb or keto diet, and leftovers freeze well for later meals.