Preheat an oven to 375℉. Coat a 9 x 13 baking dish with nonstick cooking spray and set aside.
Add the vegetable oil to a large skillet over medium high heat. Once the oil is hot, add the ground beef and cook until browned and cooked through, about 7 to 8 minutes. Add the onion and cook until translucent, another 8 to 10 minutes.
Turn the heat down to low, then add the tomato sauce, salt, pepper and garlic powder. Bring to a simmer, then cook for 4 to 5 minutes. Remove from the heat and set aside.
Using a mandoline slicer or sharp knife, slice the zucchini into thin slices lengthwise, about ⅛ to ¼ inch thick. Sprinkle both sides of each slice with salt and lay out on paper towels. Let sit for 15 minutes, then pat each slice dry with a paper towel. This will extract excess moisture so the lasagna doesn’t become soggy.
In a small bowl, whisk together the ricotta and the egg, set aside.
To assemble, arrange one third of the zucchini slices neatly along the bottom of the baking dish. It may be necessary to overlap the slices. Spread one third of the meat sauce over the top of the zucchini, followed by one third of the ricotta mixture and one third of the mozzarella. Repeat this process two more times, ending with the mozzarella.
Bake for 45 to 50 minutes, until the cheese has completely melted and the top is starting to turn golden brown. You can cover the top with aluminum foil for part of the baking time if the top begins to color too quickly. Let rest for 5 to 6 minutes before serving.