To make the Bolognese sauce, finely chop the carrots, onions, garlic and celery and add to a large, wide pan over a medium heat with a drizzle of olive oil. Roughly chop and add the bacon, then pick in the thyme leaves and cook for 5 to 10 minutes, or until softened and lightly golden.
Turn the heat up slightly, then stir in the beef mince, breaking it up with a wooden spoon. Cook for around 5 minutes, or until browned all over. Add the wine and crumble in the stock cube, stirring continuously until the liquid has completely reduced. Stir in the tomatoes and 1 tin’s worth of water and bring to the boil. Reduce to a simmer, cover and cook for around 1 hour, then remove the lid and continue cooking for 30 minutes, or until thickened and reduced.
Meanwhile, preheat the oven to 180ºC/350ºF/gas 4. Make the béchamel sauce: place the milk, butter and flours in a medium pan over a low heat. Heat gently, whisking continuously for 12 to 15 minutes, or until thickened. Grate in three-quarters of the Parmesan and a good grating of nutmeg. Season to taste, then set aside.
Spoon one-third of the Bolognese sauce into an ovenproof dish (roughly 25cm x 30cm). Layer over one-third of the lasagne sheets and top with one-third of the béchamel sauce. Repeat with the remaining ingredients until you have three layers in total, finishing with a final layer of béchamel. Grate over the remaining Parmesan and drizzle with olive oil, then cover with tin foil. Place in the hot oven for around 20 minutes, remove the foil and continue cooking for around 30 minutes, or until golden and bubbling. Serve with a nice, crisp salad.