Get your oven ready by preheating it to 400°F (204°C). Line a large baking sheet with parchment paper to prevent sticking.
Carefully slice your zucchini lengthwise into thin sheets, about 1/4 inch thick. A mandoline makes this super easy and gives you uniform "noodles"! Lay the zucchini slices in a single layer on your prepared baking sheet. Brush both sides lightly with 2 tablespoons of olive oil, then sprinkle with a pinch of sea salt.
Roast the zucchini in the oven for about 15-20 minutes, or until they're soft and most of their moisture has evaporated. This step is crucial for a non-watery lasagna! Once roasted, take the zucchini out but keep your oven hot. Gently pat the zucchini slices with paper towels to absorb any lingering moisture or excess oil.
While the zucchini roasts, heat the remaining 1 tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Add the minced garlic and sauté for just 30-60 seconds until you can smell its wonderful aroma.
Toss in the ground beef and cook, breaking it up with a spoon, for about 10 minutes until it's fully browned. Drain any excess fat if needed.
Stir in your marinara sauce. Give it a quick taste and add a tiny bit of salt and pepper if you feel it needs it (the sauce and cheeses usually add plenty!). Remove from heat.
In a medium-sized bowl, simply combine the ricotta cheese, grated Parmesan cheese, and the large egg. Stir until everything is well mixed and smooth. This creates a delightful, creamy layer.
Grab a 9x13 inch casserole dish. Lay a single layer of your roasted zucchini slices on the bottom to form your first "noodle" layer. Spread half of your rich meat sauce evenly over the zucchini. Next, dollop small spoonfuls of half of the ricotta cheese mixture over the meat sauce, then gently spread it out. Smaller dollops make spreading easier! Sprinkle half of the shredded mozzarella cheese generously over the ricotta layer.
Repeat all those layers one more time: zucchini, remaining meat sauce, remaining ricotta mixture, and finally, top it all off with the rest of the shredded mozzarella cheese.
Pop your assembled lasagna into the preheated oven. Bake for about 15 minutes, or until the mozzarella cheese on top is beautifully melted and lightly golden.
For an extra golden, bubbly crust, you can briefly broil it for a minute or two at the very end (keep a very close eye on it to prevent burning!). Let it cool for a few minutes before slicing and serving. Garnish with fresh basil if you like! Enjoy your healthy, satisfying lasagna!