The Bolognese: In a large skillet over medium-high heat, add olive oil. Once hot, sauté the diced onion for about 4-5 minutes until translucent. Add the ground beef, minced garlic, 1 teaspoon of salt, and 1/2 teaspoon of pepper. Cook until the beef is thoroughly browned, breaking it apart with a spoon as it cooks. Once browned, drain any excess grease from the skillet. Stir in the marinara sauce and let the mixture simmer on low heat while you prepare the béchamel sauce.
The Béchamel: In a medium saucepan, gently heat the heavy cream and milk over medium heat until it reaches a simmer. Then, reduce the heat to low. Gradually whisk in 1 1/2 cups of Parmesan cheese until it's completely melted and the sauce is velvety smooth. Remove the saucepan from the heat and stir in the chopped basil. Season the béchamel with the remaining 1 teaspoon of salt and 1/2 teaspoon of pepper.
Fill a large pot with water, add a pinch of salt, and bring it to a rolling boil over high heat. Gently add the lasagna noodles and cook them until they are tender but still have a bit of bite, stirring occasionally to prevent sticking, for about 8 to 10 minutes. Once cooked, drain the noodles and set them aside. If you're short on time or prefer a simpler method, feel free to use oven-ready lasagna noodles, which do not require pre-cooking.
Preheat your oven to 375°F (190°C). Grab a 9x13-inch baking dish and spread a thin layer of the bolognese sauce on the bottom. Place a layer of cooked lasagna noodles over the sauce. Spoon a layer of bolognese sauce over the noodles, followed by a generous drizzle of béchamel sauce. Sprinkle a layer of mozzarella cheese on top. Repeat these layers until all ingredients are used, finishing with a layer of noodles topped with béchamel sauce and the remaining 1/2 cup of Parmesan cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and continue baking for an additional 20 minutes, or until the top is golden and bubbly. Let the lasagna rest for about 10 minutes before slicing. Don't skip the resting time as it allows the layers to set.