Step 1: Pat the lamb dry with paper towels and season generously all over with 2 teaspoons of salt and 1 teaspoon of freshly ground black pepper.
Step 2: Heat 1 tablespoon of olive oil in a large, heavy-bottomed pan or Dutch oven over medium-high heat. Sear the lamb on all sides until deeply golden brown, about 3-4 minutes per side. Remove the lamb and set aside.
Step 3: Add the remaining 1 tablespoon of olive oil to the same pan. Add the chopped onion, carrots, and celery. Sauté for 5-7 minutes until softened, scraping up any browned bits from the bottom of the pan. Stir in the minced garlic for the last minute until fragrant.
Step 4: If using, pour in the red wine and scrape the bottom of the pan to deglaze, letting it simmer for 2-3 minutes until slightly reduced. If not using wine, use a splash of beef broth. Then, add the beef broth, tomato paste, bay leaves, fresh rosemary, and fresh thyme. Bring the mixture to a gentle simmer.
Step 5: Carefully transfer the seared lamb to your slow cooker. Pour the aromatic vegetable and liquid mixture over the lamb. Ensure the lamb is mostly submerged; add a little more broth if needed.
Step 6: Cover the slow cooker with its lid and cook on low for 6-8 hours, or on high for 3-4 hours, until the lamb is incredibly tender and easily pulls apart with a fork.
Step 7: Remove the lamb from the slow cooker and transfer it to a cutting board. Remove the bones and any excess fat, then shred the lamb with two forks. Skim any excess fat from the surface of the sauce in the slow cooker. If desired, whisk together the cornstarch and cold water in a small bowl, then stir into the hot sauce in the slow cooker. Cook on high for another 15-20 minutes until the sauce has thickened. Serve the shredded lamb with the rich sauce.