Fry the onion and garlic in the olive oil until it starts to soften, then add the peppers and continue to fry until soft - about 8 minutes all in all.
Add the beef and pancetta and continue to cook until the meat is browned.
Add the stock, wine, tinned tomatoes, balsamic vinegar, puree and herbs and stir together and then bring up to the boil.
Add the mushrooms then turn down the heat to a simmer and leave uncovered for 1 1/2 to 2 hours, or until the sauce has thickened to where you want it. Give the sauce a stir every 20 minutes or so and season to taste with the salt and pepper.
Serve with fresh pasta, garlic bread and a nice crisp salad. Top with grated Parmesan to serve.