Prepare the meat sauce: In a large pot, brown ground beef over medium-high heat. Drain any excess fat. Add diced onions and garlic, cooking until softened. Stir in tomato paste, crushed tomatoes, Italian seasoning, 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 and cook for 1 minute. Gradually whisk in milk until the sauce is smooth and thick. Stir in salt, pepper, and nutmeg. Remove from heat.
Cook the lasagna noodles: Cook lasagna noodles according to package directions until al dente. Drain and rinse with cold water to prevent sticking.
Assemble the lasagna: Preheat oven to 375°F (190°C). In a 9x13 inch baking dish, spread a thin layer of meat sauce. Layer noodles over the sauce, followed by a layer of ricotta cheese, béchamel sauce, and mozzarella cheese. Repeat layers until all ingredients are used, ending with a layer of mozzarella cheese.
Bake the lasagna: Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for another 20 minutes, or until the cheese is melted and bubbly.
Rest: Let the lasagna rest for 10-15 minutes before cutting and serving.
Serve: Cut the lasagna into squares and serve warm. Garnish with fresh basil, if desired.
