Step-1Heat 1 tsp oil in a large pan, and fry the carrot, onion and celery for 15 minutes until it is very soft and starting to caramelize. Add the mince and fry until brown all over. Add the purée and stir into the mince, then add the tomatoes with half a tin of water and the bay leaf. Simmer for 30 minutes until the sauce is thick.
Heat the oven to 180C/fan 160C/gas 4. Heat 1 tsp oil in another pan. Add the flour and cook until the oil is absorbed and the flour starts to expand. Stirring continually, add the warmed milk, a splash at a time, until completely incorporated and there are no lumps. Cook on a low heat until you have a thick white sauce, and season. Stir the spinach though the sauce, and simmer until it wilts.
Put one third of the tomato sauce in a large baking dish, and cover with a few lasagne sheets. Keep layering up until the sauce has been used, finishing with a layer of pasta. Pour over the white sauce, and scatter with the parmesan. Bake for 1 hour or until the sauce is bubbling and the top is golden. Cover with foil if it’s getting too dark. Serve with a salad.