Prepare the meat sauce: In a large skillet, brown ground beef over medium-high heat. Drain any excess fat. Add chopped onion, garlic, and bell pepper and cook until softened. Stir in tomato paste, crushed tomatoes, tomato sauce, Italian seasoning, sugar, salt, and pepper. Bring to a simmer and cook for at least 30 minutes, stirring occasionally.
Make the béchamel sauce: In a saucepan, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes, until a smooth paste forms (a roux). Gradually whisk in milk, stirring constantly to prevent lumps. Bring to a simmer and cook until the sauce thickens, about 5-7 minutes. Remove from heat and stir in nutmeg, salt, and pepper.
Cook the lasagna noodles: Cook lasagna noodles according to package directions. Drain and rinse with cold water to prevent sticking.
Assemble the lasagna: Preheat oven to 375°F (190°C). Spread a thin layer of meat sauce in the bottom of a 9x13 inch baking dish. Layer with lasagna noodles, ricotta cheese, mozzarella cheese, and béchamel sauce. Repeat layers, ending with a layer of mozzarella cheese on top.
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 and the lasagna is heated through.
Let it rest: Let the lasagna rest for 10-15 minutes before cutting and serving.
Serve and enjoy!
