Place the onions and garlic in a food processor and puree.
Place the beef in a zip lock bag, add the onion mixture, salt and pepper.
Be sure the beef is covered with the marinade and refrigerate for at least 3 hours or overnight.
In a large pot over medium-high heat, melt the butter and brown the meat on all sides.
Add beef broth and wine to cover the beef and cook for 2 hours over medium heat.
Add the brown sugar, Coca-Cola, butter and Worcestershire sauce.
Cook for about 45 minutes more. To serve slice the beef and pour the sauce on top.
Notes
*Use a good red wine to cook the beef.*I prefer to marinade the beef overnight.*I recommend using a good-quality beef broth.*Check the meat after two hours of cooking if the sauce is getting dry add one cup of water to the pot and continue cooking.*You can store this posta negra in the fridge up to 3 days.