1. Using a small sharp knife, cut 20 small pockets into the lamb leg. Poke the garlic slivers into these pockets.
2. In a small bowl, mix the wholegrain mustard and the honey together.
3. Spoon some of this mixture into the pockets and pour the remainder over the whole leg. Poke rosemary sprigs into all the pockets.
4. Place an upturned saucer into the slow cooker to act as a trivet. Or alternately you can place the lamb on a base of onions, potatoes and carrots. Place the lamb on this and pour the water around it. Cook on low for 8 hours.