Melt the butter in a large pot over medium heat, and cook the onion until softened. Add the stock, raise the heat to high, and bring to a boil. continue boiling for 5 minutes. Whisk in the whole jar of mustard, bring to a boil again, and reduce the heat to medium. Cook at a low boil for another 15 to 20 minutes, stirring occasionally. Whisk in the cream and bring to a boil again. season with salt and pepper. if the mustard flavor is weak, continue to boil over medium heat until soup is slightly reduced and the mustard taste is stronger. stir in the parsley just before serving. ladle soup from bottom of the pot, which is where the mustard seeds has sunk.