Season the roast on all sides by sprinkling 2 tsp of salt and 2 tsp of pepper over the entire roast. Sauté the diced onion in a little oil until golden brown and set aside. Heat a Dutch oven on high heat and add oil. When the oil is hot, place the roast in a pot and cook until it is browned on all sides. Add the beef broth, cola, garlic, sautéed onion, rosemary, and remaining tsp of salt. Bring to boil, stirring to release browned bits. Add the tomato paste and stir. For the crock-pot cooking method, transfer meat and juices to crock-pot and cook on high for about 4-5 hours (or until fork tender) or cook overnight in crock-pot on low heat. For the oven method, cover Dutch oven and cook at 325ºF in the oven for 3 ½ hours. Liquid should be at least ⅓ of the way up the roast. Add more beef broth if necessary.