Place silverside into the bowl of a 5 litre slow-cooker. Add herbs, 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. Serve with steamed beans and mustard sauce.
To make mustard sauce, heat medium 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.