Preheat your oven to 375°F (190°C).
Cook the lasagna noodles according to the package instructions until al dente. Drain and set aside.
In a skillet over medium heat, brown the ground beef until fully cooked. Drain excess fat if necessary.
Add the marinara sauce to the cooked beef, stirring well to combine. Simmer for 5 minutes.
In a saucepan, melt the butter over medium heat. Once melted, stir in the flour to create a roux, cooking for 1-2 minutes.
Gradually whisk in the whole milk, continuing to cook while stirring until the mixture thickens and comes to a simmer.
Stir in the grated Parmesan cheese, salt, black pepper, and nutmeg into the béchamel, mixing until smooth.
Spread a thin layer of marinara sauce on the bottom of a baking dish.
Layer 4 lasagna noodles on top of the sauce, followed by half the meat sauce, half the béchamel, and a sprinkle of mozzarella.
Repeat the layering process with another set of noodles, meat sauce, béchamel, and mozzarella.
Top the final layer with remaining noodles, béchamel, and mozzarella. Add extra Parmesan if desired.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes or until the top is golden and bubbly.
Let the lasagna rest for at least 10 minutes before slicing and serving.