Begin by preheating your oven to 375°F (190°C) to ensure it's hot when you're ready to bake your lasagna.
In a large skillet, heat a drizzle of olive oil over medium heat. Add the chopped onion and minced garlic, sautéing them together until the onion becomes translucent, about 3-4 minutes.
Add the ground beef or turkey to the skillet. Season with salt, pepper, and the Italian seasoning. Cook the mixture, breaking it apart with a spatula, until the meat is browned and cooked through, approximately 6-8 minutes. If there's excess fat, drain it from the skillet.
Pour in the marinara sauce, stirring it into the meat mixture. Reduce the heat and let it simmer for about 5 minutes, allowing the flavors to meld.
While the sauce is simmering, in a mixing bowl, combine the ricotta cheese, egg, half of the grated Parmesan cheese, and a pinch of salt and pepper. Stir the mixture until smooth and creamy.
Spread a thin layer of the savory meat sauce on the bottom of a greased 9x13 inch baking dish.
Lay down 3 lasagna noodles over the sauce, ensuring they are evenly spaced. Spread half of the ricotta mixture over the noodles, then layer on half of the meat sauce. Top this with 1 cup of the shredded mozzarella cheese.
Repeat the layering process: Add another 3 lasagna noodles, the remaining ricotta mixture, the rest of the meat sauce, and half of the remaining mozzarella cheese.
Finish the lasagna assembly with the last 3 lasagna noodles on top, followed by the remaining meat sauce, the rest of the mozzarella, and the remaining grated Parmesan cheese.
Cover the baking dish tightly with aluminum foil to trap the steam while it cooks. Place it in the preheated oven and bake for 25 minutes.
After 25 minutes, carefully remove the foil and continue baking for an additional 15-20 minutes, or until the cheese is deliciously bubbly and golden on top.
Once done, let the lasagna sit for about 10 minutes before slicing into squares for serving. This resting time helps the layers set. Garnish with fresh basil leaves for a pop of color and flavor, if desired.