Heat a frying pan over medium heat. Add chopped onion and cook until soft. Add garlic and cook for one minute. Add ground beef and cook until browned. Stir often.

Add chopped tomatoes and tomato paste to the beef. Stir well. Add oregano, salt, and pepper. Let it simmer for 10 minutes. The mixture should thicken slightly.

In a saucepan, melt butter over medium heat. Add flour and stir for one minute. Slowly add milk while whisking. Cook until thick and smooth. Add salt and nutmeg.

Preheat your oven to 180°C (350°F). Lightly grease your baking dish. This prevents sticking and helps easy serving later.

Spoon the beef mixture into the cannelloni tubes. You can use a small spoon or piping bag. Fill carefully but do not overstuff.

Place filled cannelloni in a single layer inside the baking dish. Make sure they are close but not squashed together.

Pour the white sauce evenly over the cannelloni. Make sure all pasta is covered. This keeps the pasta soft while baking.

Sprinkle mozzarella and Parmesan cheese on top. The cheese will melt and create a golden crust.

Bake in the preheated oven for 35–40 minutes. The top should be golden and bubbling. Let it rest for 10 minutes before serving.