1. Heat 2 Tablespoons of oil in a pot and add the sliced onions. Cook onions slowly until soft, stirring occasionally. Increase the heat to medium high and add the wine, thyme and beef stock. Bring to a boil and add the Worcestershire sauce, ketchup and mustard. Reduce the heat to low and cook until the mixture is thick and caramelized. Season with salt and pepper if desired. Keep warm if using immediately or chill for later use.
2. Gently form the meat into four patties. Brush each side with mustard and coat evenly with the peppercorns. Keep chilled until ready to cook.
3. Heat 2 Tablespoons olive oil in a pan. Add the burgers and cook for six to seven minutes, turning once or twice, for medium rare. Continue cooking to desired degree of doneness. Split and toast the rolls and put lettuce and tomato on the bottom half if desired. Add the burgers and top with the onion mixture and top of roll.