Preheat oven to 375 degrees. Prepare a 9x11 glass dish with a thin layer of marinara sauce on the bottom. Save the rest of the sauce for later.
Drizzle olive oil in a large skillet over medium-high heat. Once hot, add as many thin strips of zucchini as you can fit into the pan without overlapping. Season with salt. Cook for 1-2 minutes per side (longer for thicker pieces and shorter for thinner pieces). Set on a plate with paper towels to soak up the excess moisture. Continue to cook the zucchini in batches. Once all of the zucchini are cooked, each layer should go between a new paper towel, so the lasagna does not become watery.
In the same pan, add the ground beef and onion, breaking apart and cooking until fully done about 8 minutes. Drain excess grease. Turn off the heat and add the marinara sauce to the ground beef and onion mixture.
Now it's time to make your lasagna! Start with a single layer of zucchini on top of the thin layer of marinara sauce already on the dish's bottom. Spread 1/3 of the ricotta on top of the zucchini, 1/3 of the parmesan cheese, and 1/3 of the mozzarella cheese. Finally, add 1/3 of the beef sauce. Layer exactly the same way one more time. Once you get to the zucchini's final layer, follow a slightly different pattern. Add the rest of the ricotta, the rest of the parmesan cheese, and the rest of the sauce in that order. Top with the rest of the mozzarella.
Place in the oven uncovered and cook for 40 minutes. Let rest for a couple of minutes then serve!