Heat a skillet over medium heat and brown the lamb chunks on all sides for about 5 minutes. Transfer the lamb to the slow cooker.
Add the lentils, onion, carrots, garlic, rosemary sprigs, diced tomatoes, beef broth, salt, and black pepper to the slow cooker.
Stir everything to combine, then cover and cook on low for 7 to 8 hours until the lamb is tender and the lentils are soft.
Remove and discard the rosemary sprigs before serving.
Ladle the stew into bowls and serve with crusty bread on the side.