Preheat the oven to 180 °C. Heat a large frying pan for the ragu and add the oil. Put the ground meat in the pan and fry it until it gets a golden brown crust. Take the pan from the heat source and scoop the meat on a plate. Put the onion, celery (if you use it) and garlic in the pan and fry until everything is soft. Put the meat back in the pan and stir in the flour. Add the stock and bring to a boil. Add the sugar, tomato paste and thyme and stir well.
Stir in the Passata. Bring back to the boil, cover and simmer in the oven for 1 to 1 1/2 hours or until the meat is tender.
Melt the butter in a saucepan for the white sauce. Add the flour and stir well to avoid lumps. Let it cook for a while, so that the flour flavour disappears completely. Incorporate the warm milk little by little with continuing to beat until the sauce thickens. Add the Dijon mustard and parmesan and season well with pepper and salt.
For the lasagna, divide a third of the meat sauce over the bottom of a deep 2.3 litre oven-proof dish. Spoon a third of the white sauce. Arrange a layer of lasagna sheets on it. Season with pepper and salt. Spoon half of the remaining meat sauce, followed by half of the white sauce. Put back a layer of lasagna sheets here. Finish with the remaining meat sauce and the remaining white sauce. Sprinkle the cheddar on top.
Place the dish in the middle of the oven for about 45 minutes or until the lasagna turns golden brown, bubbles at the corners, and the pasta is soft.