Add the lamb stew meat to the slow cooker.
Place the cubed potatoes and sliced leeks on top of the lamb.
Pour the stout beer and beef broth over the ingredients.
Add the minced garlic, rosemary sprigs, dried thyme, salt, and black pepper.
Stir gently to combine everything, then cover the slow cooker.
Cook on low for 8 hours until the lamb is fork-tender and the potatoes are soft.
Remove the rosemary sprigs before serving.
Ladle the stew into bowls and sprinkle with chopped fresh parsley.