Set the slow cooker to low.
Add prepared vegetables to the slow cooker and place corned silverside on top.
Pour in enough hot water to cover meat. Add vinegar, golden syrup, bay leaves and peppercorns.
Cook on low for 8-10 hours.
Add mustard powder to White Sauce Mix and make up as directed on the packet.
Slice corned silverside and serve with the sauce and potatoes with fresh vegetables.