Heat the oil in a heavy-bottomed saucepan over a medium-high heat and fry the onion, celery and carrot for 8-10 minutes, until soft. Add the garlic and fry for 1 minute, then add the mushroom mince, thyme, bay leaf and rosemary and cook for another 5 minutes.
Add the tomato paste and cook, stirring, for 2 more minutes. Pour in the stock and chopped tomatoes, seasoning well. Simmer over a low to medium heat for 20 minutes until the sauce has thickened slightly.
Cook the pappardelle according to the instructions on the packet. Serve with the ragu and a sprinkling of cheese on top.