Put the vegetables in a large, heavy-based saucepan and cover with 500ml/18fl oz water. Bring to the boil, then simmer for 8-10 minutes, or until the onion and pepper are very soft. Remove from the heat and blend the contents to a smooth purée using a hand-held blender. Set aside and keep warm.
Heat a separate, non-stick, lidded saucepan over a medium heat. Dry-fry the beef mince for 3-5 minutes, stirring with a wooden spoon to break up any large clumps, until lightly browned on the outside.
Add the puréed vegetables, chopped tomatoes, tomato purée and dried herbs to the pan, stirring well to combine. Crumble over the stock cube and add the bay leaf. Season with pepper and stir again.
Bring to the boil, then reduce the heat until it is simmering. Half-cover the pan with the lid and continue to simmer gently for 25-30 minutes, stirring the sauce more and more regularly as it thickens.
Meanwhile, bring a large pot of salted water to the boil. Add the spaghetti and cook until al dente.
Divide the spaghetti between bowls and top with the Bolognese. Season with black pepper and Parmesan.