Heat the olive oil in a large pan and gently cook the onions with a pinch of salt until very soft. This will take approximately 10 minutes.
Stir in the garlic and prosciutto and cook for 3 minutes.
Add the diced beef and pork and cook over a medium heat until brown all over.
Pour in the wine and simmer until the wine has almost completely reduced.
Add the tomatoes, beef broth, dried herbs, optional parmesan rind, salt and pepper, cover and gently simmer over a low heat for 3-4 hours, until the meat is falling apart and the sauce has thickened. Stir occasionally, adding more broth or water if necessary. If the ragu has too much liquid at the end of the cooking time take off the lid and simmer until it has thickened.
Add the rosemary and cook for a few more minutes, taste the sauce for salt and pepper and stir in the butter. Remove the parmesan rind (if using) and serve the ragu over your favourite pasta with a sprinkle of grated parmesan.