Preheat your oven to 375°F (190°C).
Heat a large skillet over medium heat and add a drizzle of olive oil. Sauté the onion for about 3-4 minutes until softened, then add the minced garlic and cook for an additional minute.
Add the ground turkey or beef to the skillet. Cook until browned, breaking it into small pieces, about 5-7 minutes. Drain any excess fat if necessary.
Stir in the crushed tomatoes, oregano, basil, red pepper flakes (if using), salt, and black pepper. Simmer the sauce for about 10 minutes, allowing the flavors to meld.
In a mixing bowl, combine the ricotta cheese, egg, and a pinch of salt and black pepper. Mix well until smooth.
To assemble the lasagna, spread a thin layer of the meat sauce on the bottom of a 9x13 dish. Add a layer of zucchini slices, followed by dollops of the ricotta mixture, a sprinkle of mozzarella, and more sauce. Repeat the layers until all ingredients are used, finishing with a layer of meat sauce and the remaining mozzarella cheese on top. Sprinkle grated Parmesan over the top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 20 minutes, or until the cheese is bubbly and golden.
Allow the lasagna to cool for 10-15 minutes before slicing. Serve warm.