Preheat oven 220°C fan forced. Lightly grease a large, deep ovenproof dish.
Heat oil in a medium, deep frying pan over medium heat. Add onions and garlic. Cook, stirring occasionally for 6 minutes, or until onion is soft. Increase heat to high, add mince. Cook, stirring to break up mince, for 4 minutes, or until browned.
Add tomatoes and sugar. Bring to boil. Gently boil, stirring occasionally, for 15 minutes, or until sauce thickens slightly. Stir in basil. Season.
Meanwhile, cook pasta in a large saucepan of boiling, salted water for 3 minutes. Drain.
For béchamel sauce, melt butter in a medium saucepan over medium-high heat until foaming. Add flour. Cook, stirring for 2 minutes, or until bubbling. Remove from heat. Slowly add milk, whisking constantly, until smooth. Return to the heat. Cook, whisking until sauce comes to the boil. Remove from heat. Stir in nutmeg, season.
To assemble, spoon half the pasta into the prepared dish. Spoon over half the beef mixture. Spoon over half the béchamel sauce. Repeat with remaining pasta, beef mixture and béchamel sauce. Top with mozzarella.
Place onto a tray and bake for 25 minutes, or until cheese is golden. Sprinkle with extra basil leaves and pepper. Serve.