Put some oil in a frying pan over medium heat. Give the beef a good sear on all sides (we're looking for a nice brown color)
In a slow cooker, place the beef and fill the slow cooker with the beef stock. Add in your tomato puree, parsley, rosemary, and peppercorns.
Place the slow cooker on high heat for 4 hours. Be sure to flip the beef every half hour so that the entire cut cooks evenly.
Remove the beef when it is extremely tender. A good way to know that it's cooked perfectly it to take a pair of tongs and attempt to pinch them at the center of the meat. If you can pinch the tongs without using much pressure, your beef is good to go! Place the beef in a stainless steel bowl and use your tongs and a fork to pull the meat apart.
Place a pot on your stove with a strainer over it. Empty the beef stock from the slow cooker into the pot, using the strainer to remove the parsley, rosemary, and peppercorns from the stock.
Reduce the stock over low heat. You want to remove some of the water from the stock to make it richer. A trick to knowing when it's sufficiently reduced is to take a spoon and dip it into the stock. If there's a noticeable coating around the spoon, the stock has been reduced!
Pour the reduced stock into the stainless steel bowl a little bit at a time and mix it into the shredded beef. Season with salt and pepper to taste, and you have yourself a simple but very delicious braised beef!