Pat the meat dry with paper towels and sprinkle black pepper on all sides. Sear in a hot skillet on the stove on all sides. Transfer to a 4 to 5-quart slow cooker.
In a medium bowl, whisk together the soy, Worcestershire, sugar, garlic, ginger, and vinegar. Pour over meat in the slow cooker. Cover and cook on low heat for 6-8 hours or on high for 4 hours.
In a small bowl, whisk together cornstarch and 2 tablespoons of water. Stir cornstarch mixture into the slow cooker. Cover and cook on high heat for an additional 30 minutes, or until the sauce has thickened.
This is great over rice or noodles. It's also good in sandwiches and tacos.