Heat the olive oil in a large skillet over medium-high heat. Add the lamb cubes in batches and brown them on all sides, then transfer to the slow cooker.
Add the diced onion and minced garlic to the same skillet. Cook for 3 minutes until the onion softens.
Pour the beef broth into the skillet and scrape up any browned bits. Transfer the mixture to the slow cooker.
Add the cherry tomatoes, chopped sage, bay leaves, salt, and black pepper to the slow cooker. Stir gently to combine.
Cover and cook on low for 7 hours until the lamb is fork-tender.
Remove the bay leaves. Taste and adjust salt if needed.
Ladle into bowls and sprinkle with chopped parsley before serving.