Clean and rinse then pat the lamb dry with paper towels. Place lamb in the slow cooker (make sure that the lamb fits inside the slow cooker before you cook).
Rub salt & pepper all over the lamb. Drizzle olive oil on top of the lamb. Add garlic, thyme and rosemary on top of the lamb. Pour in beef broth.
Slow cook on low for 10 hours or until tender (cook on high for 5 hours).
Transfer the cooked lamb on to a plate and cover until ready to eat. Transfer the broth from the cooker to a cup.
Place butter in the saucepan and turn it up to medium-high. Wait for 30 seconds until it melts. Add flour and stir constantly for 30 seconds. Slowly pour ½ cup of broth into the pan; whisk until smooth. Repeat the step by adding second ½ cup; whisk until smooth. Then slowly pour in the rest of the broth; mix and whisk. Reduce the heat to medium and let it simmer for 10 minutes or until desired consistency. Season with salt and pepper as needed. Optional: strain the gravy to remove any chumps of flour.