Rub lamb all over with olive oil, rosemary and garlic then sprinkle with salt and pepper.
Place lamb in the slow cooker and pour the beef stock around the lamb (not over the top) and cook for 10 hours on low setting (refer to your slow cooker's instructions).
Towards the end of the lamb's cooking time, preheat oven to 200c.
When it's finished, take the lamb out of the slow cooker and place it in a baking dish (be careful as you transfer it, it will be very soft) and pour the cooking juices into a jug, skim off any fat and set aside to make the gravy.
Bake lamb uncovered for 20 minutes until browned on top.
Melt butter in a saucepan until starting to foam.
Add plain flour and mix to combine, then stir over low heat for 1 minute.
Switch off the heat, then add cooking juices in approximately ¼ cup amounts (stir well with each addition until there are no lumps before adding the next ¼ cup - see my Gravy Making Notes below), once this mixture is the consistency of cream you can add the remaining cooking juices. Stir over low heat until simmering and thickened, adding salt and pepper to taste.
Slice or shred the lamb (discarding fat and bones) then serve with gravy.
