Preheat the oven to 190C/180C Fan/Gas 5.
Put the beef mince in a colander and rinse under the cold tap to separate it into smaller pieces. Drain well and pat dry with kitchen paper.
Transfer the mince to a roasting tray and roast for 35-60 minutes, or until completely crisp and golden-brown. This intensifies the flavour and helps it to absorb the sauce ingredients. Drain off the fat.
Heat the oil in a large, heavy-based casserole over a medium heat. Add the bacon and fry for 4-5 minutes, stirring from time to time, until the fat melts and the bacon starts to brown.
Add the onion, celery, carrots, garlic and bay leaf and cook for 4-5 minutes, or until they begin to soften.
Stir in the vinegar, then the sugar and oregano. Add the cooked mince, tomatoes, stock and mushrooms. Stir well and bring to the boil, then reduce the heat and simmer for 45-60 minutes, stirring occasionally, until thickened. Season, to taste, with salt and pepper, then set aside.
Meanwhile, cook the spaghetti in a pan of boiling, salted water, stirring often with a fork, until just tender (al dente). Drain immediately in a colander, shaking it.
To serve, twirl the spaghetti onto 4 plates. Spoon over the Bolognese sauce and garnish with Parmesan, if desired.