In 4 to 6 qt slow cooker place the sliced onions in the bottom. Top the onions with the pork roast. Season the pork roast with salt and pepper.Next, in a medium mixing bowl combine tomato sauce, garlic, molasses, white vinegar, whole grain mustard, oregano, cumin, and pickle relish. Pour the mixture over the pork roast.Set the slow cooker on high for 4 hours or on low for 8 to 10 hours.Once cooked, remover the pork roast and shred with fork. Serve the shredded pork with buns or pickles if desired!