Place silverside into the bowl of a 5 litre slow-cooker. Add sweet potato, parsley and bay leaves, garlic, onion, celery, pumpkin, potatoes, carrot, vinegar, water and peppercorns.
Cover with lid. Turn slow-cooker on low. Cook for 8 hours or until silverside is tender.
Remove silverside from cooking liquid and transfer to chopping board. Reserve ¼ cup (60ml) cooking liquid from slow-cooker for mustard sauce. Cover meat with foil. Stand 10 minutes before slicing thinly.
To make mustard sauce, place frying pan over medium-high heat. Add butter, then cooking liquid, thyme and mustard. Add cream. Bring to the boil and simmer for 5 minutes or until sauce thickens slightly. Serve with steamed beans and mustard sauce.
