
Prepare the meat sauce: In a large pot, brown ground beef over medium-high heat. Drain excess fat. Add chopped onions, garlic, carrots, and celery; cook until softened. Stir in tomato paste, crushed tomatoes, oregano, basil, salt, and pepper. Simmer for at least 30 minutes, stirring occasionally.
Make the béchamel sauce: In a saucepan, melt butter over medium heat. Whisk in flour until smooth. Gradually add milk, whisking constantly to prevent lumps. Cook until the sauce thickens, then stir in nutmeg, salt, and pepper.
Cook the lasagna noodles according to package directions. Drain and set aside.
Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Assemble the lasagna: Spread a thin layer of meat sauce in the bottom of the dish. Top with a layer of lasagna noodles, followed by a layer of ricotta cheese, a layer of béchamel sauce, and a sprinkle of mozzarella cheese. Repeat layers until all ingredients are used, ending with a layer of meat sauce and mozzarella cheese.
Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 20 minutes, or until the cheese is melted and bubbly.
Let the lasagna rest for 10-15 minutes before slicing and serving.