Cook the macaroni as pack instructions until tender. Drain.
Meanwhile, fry the minced beef and onions until brown. Stir in the tomatoes, tomato puree, herbs and seasonings. Bring to the boil, cover and simmer for 15 minutes.
To prepare the topping, melt the butter in a pan, add the flour and cook for 1 minute, stirring. Blend in the milk and bring to the boil, still stirring. As soon as it thickens remove from the heat. Be careful not to burn it. Stir in 100g of the cheese, the yoghurt and the beaten egg.
Combine the macaroni and minced beef mixture and put into an ovenproof dish. Pour over the topping and sprinkle with the remaining cheese. Bake for 35-40 minutes until golden. Stand for 5-10 minutes before serving to allow the topping to firm up.