Make small incisions all over lamb with a very sharp knife, then stuff each with a sliver of garlic and a sprig of rosemary.
Heat oil in a large frying pan, cook lamb until browned all over. Remove from pan and transfer to slow cooker.
In the same pan, add butter and cook onion 1-2 minutes or until softened, place in slow cooker with remaining ingredients. Season with salt and pepper.
Cover and cook on HIGH 3-4 hours, or LOW 6-8 hours.
Remove meat from slow cooker, and rest for 10 minutes before carving. Serve slices of lamb with beans and parsley sauce.