Heat the oil in a large pan over a high heat. Brown the lamb in batches until properly golden.
Drop the heat slightly, add the onion, carrot, and celery and cook for 5 minutes until soft.
Stir in the garlic and cook for 30 seconds. Add the tomato paste and cook it out for 2 minutes.
Pour in the wine, scrape up the sticky bits, let it reduce for a couple of minutes.
Tip everything into the pressure cooker (or slow cooker). Add the chopped tomatoes, stock, rosemary, and add the lamb.
Pressure cooker: cook on high for 45 minutes, then let it release naturally for 15 minutes.
Slow cooker: low for 7 to 8 hours or high for 4 to 5 hours until the lamb is collapsing.
Lift out the lamb, shred it with two forks, remove the rosemary.
Return the lamb to the sauce and simmer until thick, glossy, and spoon-coating. Stir in the butter and adjust the seasoning.
Cook the pappardelle in salted boiling water until just al dente. Reserve a mug of pasta water.
Add the pasta straight into the ragu with a splash of pasta water, add the chopped parsley, and then toss until the sauce is glossy. Dish it up and finish with grated parmesan.