Use a 6-quart slow cooker. Place the meat into the slow cooker and add the onions, separating the rings with your fingers. Add the tomato paste, brown sugar, vinegar, Worcestershire sauce, chili powder, and salt. Stir well to combine. The mixture will be quite thick and will look dry. Do not add water.
Cover and cook on low for 8-10 hours, or until the meat has begun to break down and absorb the sauce. Serve on soft buns with your favorite toppings or in large romaine lettuce leaves.