Heat the oil in a large heavy-based pan over a low to medium heat. Cook the onion for 5 minutes until softened, then turn the heat up and add the minced beef. Continue to cook until browned, then add the crushed garlic. Cook for a couple of minutes, but do not allow the garlic to brown.
Add the pepper and mushrooms and cook for a further 5 minutes until they begin to soften. Add the olives, tinned tomatoes, 300ml of the beef stock, tomato purée and dried oregano and bring to a simmer. Allow to simmer for 40 minutes, topping up with more stock if it starts to dry out, until the sauce has thickened. Season with black pepper.
Cook the pasta according to the packet instructions and serve with the bolognese. Sprinkle with grated parmesan to serve.