Melt 1 tbsp butter in a skillet over medium heat. Add onions and toss to coat in butter. Cook 15-20 minutes, stirring occasionally. Add sugar and Worcestershire and cook for 2 minutes. Stir in vinegar, 1 tbsp butter, then reduce heat, cover, and simmer for 20-25 minutes, stirring every 5 minutes or so. As the sauce reduces, add beef stock 2 tablespoons at a time to keep the pan wet and the sugars from burning. Don’t worry if some brown bits end up on the bottom of the pan! This is a delicious flavor that will scrape up every time you add the stock, so take the time to scrape the pan with each addition. Add salt and pepper, stir, then serve, topping with chopped parsley.