Heat the oil in a large saucepan over a medium heat then add the onion and cook and stir until soft, about 4 to 6 minutes. Add the the garlic and then the beef, a handful at a time, until browned. Add the mushrooms, red pepper, celery, carrot and dried herbs and cook for another couple of minutes.
Stir in the tomatoes, beef stock, tomato purée and seasoning. Bring to the boil, then reduce the heat, cover and simmer, stirring occasionally, for 15 to 20 minutes.
When the sauce has been cooking for 10 minutes, start to cook the spaghetti. Bring a large saucepan of water to the boil then add the spaghetti and cook for 8 to 10 minutes until tender. Drain.
Season the bolognese sauce with pepper. Plate the spaghetti and serve with the sauce, adding Parmesan cheese on top to taste.