In a pressure cooker, cook bacon over medium heat until crisp; drain. Add potatoes and onions. In a bowl, combine the vinegar, water, sugar, 2 tablespoons of parsley, salt, mustard and pepper; pour over potatoes., Close cover securely; place pressure regulator on vent pipe. Bring cooker to full pressure over high heat. Reduce heat to medium and cook for 5 minutes. (Pressure regulator should maintain a slow steady rocking motion or release of steam; adjust heat if needed.) Remove from the heat. , Immediately cool according to manufacturer's directions until pressure is completely reduced. Just before serving, sprinkle with remaining parsley.