Prepare the garlic rosemary butter by mixing 6 tablespoons softened unsalted butter, 6 minced garlic cloves, and 2 tablespoons finely chopped fresh rosemary in a small bowl. Set aside.

Pat the leg of lamb dry with paper towels. Trim excess fat if desired, then score the surface in a crisscross pattern with a sharp knife.

Spread the garlic rosemary butter generously all over the lamb, rubbing it into the scoring. Season liberally with salt and freshly ground black pepper.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Sear the lamb on all sides until golden brown, about 2-3 minutes per side.

Transfer the seared lamb to the slow cooker. Pour 1 cup of dry red wine or beef broth around the lamb. Add optional herbs like thyme or bay leaf.

Cover and cook on low for 6 to 8 hours until the lamb is fork-tender and reaches an internal temperature of about 145°F (63°C) for medium-rare to medium.

Remove the lamb from the slow cooker and let it rest loosely covered with foil for 15-20 minutes to allow juices to redistribute.

Slice the lamb against the grain, spoon over any remaining garlic rosemary butter from the slow cooker, and serve.