Cook the lasagna noodles according to the package instructions. Drain and set aside.
In a large skillet, cook the ground beef over medium-high heat until it is browned. Add the chopped onion and minced garlic and cook for another 2-3 minutes, until the onion is translucent.
Add the can of crushed tomatoes, dried basil, and dried oregano to the skillet. Stir everything together and season with salt and black pepper to taste. Bring the sauce to a simmer and cook for 20-30 minutes, until it has thickened and the flavors have melded together.
In a small saucepan, melt the butter over medium heat. Whisk in the all-purpose flour to make a roux. Gradually whisk in the milk, stirring constantly, until the mixture has thickened. Add the ground nutmeg and grated Parmesan cheese and stir everything together.
Preheat the oven to 375°F.
To assemble the lasagna, spread a thin layer of meat sauce in the bottom of a 9x13 inch baking dish. Add a layer of lasagna noodles on top of the sauce. Spread a layer of bechamel sauce over the noodles, and then sprinkle some shredded mozzarella cheese over the bechamel. Repeat the layers until you've used up all the ingredients, ending with a layer of meat sauce and shredded mozzarella cheese on top.
Cover the lasagna with foil and bake for 25 minutes. Remove the foil and bake for another 25 minutes, until the cheese is melted and bubbly.
Let the lasagna cool for 5-10 minutes before slicing and serving.
Notes
Slow Cooker Instructions:Lasagna can be made in a slow cooker by layering the ingredients in the same way as the oven-baked version. Cook on low for 4-6 hours or on high for 2-3 hours.