Heat the oil in a large frying pan or saucepan. Sauté the onion and carrot until softened. Add the garlic and sauté for 1 minute.
Add the beef gradually pressing any lumps out with a fork. Cook until lightly coloured. Add the passata, tomatoes, tomato paste and herbs. Stir well and bring to a simmer. Season. Cover and cook for about 30 minutes or until thick.
Meanwhile, prepare the bechamel sauce. I prefer to do this in the microwave as the sauce can often stick and burn in a saucepan.
Microwave the butter in a large microwave-proof bowl until melted. Stir in the cornflour until smooth. Gradually stir in 1 cup of milk. Microwave for 2 minutes until hot. Stir well. Cook until thick then stir in the remaining milk until smooth. Microwave again until thick. Stir well. Stir in the parmesan until melted.
Preheat the oven to 180°C.
Lightly grease a 28cm x 20cm x 7cm baking pan. Spread 1 cup of the meat sauce on the base. Top evenly with 3 sheets of the lasagne. Cover with 2 cups of the meat sauce, half the bechamel and half the mozzarella. Top with the remaining lasagne sheets, meat sauce and mozzarella.