Boil pasta in salted water until just tender. Drain and set aside.

Heat olive oil in a skillet over medium heat. Add chopped onion and cook until softened. Add bell pepper and cook 2–3 minutes.

Add ground beef and cook, breaking it up, until no longer pink.

Stir in salt, black pepper, Italian seasoning, garlic powder, and oregano.

Add tomato sauce, stir well, and simmer a few minutes until thickened.

Make the white sauce: melt butter in a saucepan. Stir in flour until smooth, then slowly whisk in milk. Cook, stirring, until thick. Season with salt, black pepper, and nutmeg. Stir in 50 g mozzarella until melted.

Assemble in a baking dish: spread a layer of white sauce, add pasta, spoon on beef sauce, add more pasta, then finish with remaining white sauce.

Top with 150 g mozzarella and bake until melted and golden. Rest 5–10 minutes before serving.