Heat the olive oil in a large saucepan or casserole dish, and fry the carrot, onion, celery and peppers for 15 minutes over a medium heat until they are very soft and starting to caramelise, stirring occasionally. Add the mince and fry, for 5 minutes, until brown all over. Stir to break the mince up among the vegetables.
Add the tomato purée and stir into the mince, then add the chopped tomatoes with half a tin of water and the bay leaf. Bring to the boil and then reduce the heat to a steady simmer, cook for 30 minutes, until the sauce is thick. Heat the oven to 180C/fan 160C/gas 4. Season the tomato sauce, to taste, with salt and freshly ground black pepper. Remove the bay leaf!
To make the white sauce, melt the butter on a medium to low heat in a medium saucepan and then add the flour and cook for 1 minute. Stirring continually, add the milk gradually until completely incorporated and there are no lumps (I like to use a wooden spurtle). Turn the heat to low and add the 35g of grated parmesan and stir until it's melted into the sauce. Stir the spinach through the sauce until it wilts. Season with some freshly ground black pepper.
Now assemble the lasagne - put half of the tomato sauce in a large baking dish (I used a rectangular Staub dish - dimensions: 27cm x 20cm), and cover with 3 dried lasagne sheets. Layer the rest of the sauce on top and the finish with the other 3 dried lasagne sheets. Pour over the white sauce, and scatter with the remaining 30g of grated 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 (I top ours with foil for the last 15 to 20 minutes).