Heat a large, heavy-based pan. Add the oil and sauté the pancetta or bacon for a minute or two until it has begun to crisp and release some of its fat. Add the onion, carrot, celery, garlic, thyme and bay leaf. Cook for 8-10 minutes until the vegetables have softened and taken on a little colour, stirring occasionally.
Add the minced beef to the pan and mix until well combined, then sauté until well browned, breaking up any lumps with a wooden spoon. Stir in the tomato puree and continue to cook for another minute or two, stirring.
Deglaze the pan with a little of the wine, scraping the bottom of the pan to remove any sediment. Pour in the remaining wine with the tomatoes and then add the anchovy essence or anchovies and season to taste. Bring to the boil, then reduce the heat to the lowest setting and simmer for at least 2 hours, or up to 4 hours is best until the beef is meltingly tender and the sauce has slightly reduced.
When ready to serve, bring a large pan of water to a rolling boil. Add a good pinch of salt and swirl in the spaghetti. Stir once and then cook for 8-10 minutes or according to instructions on the packet until the pasta is al dente. Drain and quickly refresh under cold running water, then return to the pan and add the butter and Bolognese sauce. Toss until well combined and then divide among warmed wide-rimmed serving bowls. Scatter over some Parmesan to serve.