Place the silverside in the centre of a roasting pan. Arrange the potatoes and carrots around the sides. Place the onion, garlic, crumbled bay leaf and mustard seeds on top of the roast then season with salt and pepper. Pour enough water into the pan to almost cover the potatoes. Cover with a lid or aluminium foil.
Roast for 5 to 6 hours in the preheated oven until the roast is so tender it can be flaked apart with a fork.