Prepare the beef. Trim excess surface fat and cut the beef into large, even chunks.
Layer the ingredients. Place the beef in a large, deep skillet or heavy-bottomed pot. Top evenly with sliced onions, then add pieces of butter and pour in the water.
Cover with a lid and bring to a gentle simmer over medium-low heat. Reduce the heat to low and cook for 1 hour, allowing the beef to slowly tenderize and release its juices.
Add garlic and seasonings. Remove the lid and add the chopped garlic, salt, sugar, black pepper, and vinegar. Stir gently to combine.
Cover again and continue cooking over low heat at a gentle simmer for about 1 hour, or until the beef is fork-tender and the sauce is silky and glossy.
Finish with parsley. Finely chop the fresh parsley and sprinkle it over the garlic butter beef just before serving.