Place beef in a large saucepan with carrots, onions, juniper berries, peppercorns, parsley, bay leaves and thyme. Cover with cold water, add vinegar and bring to the boil. Reduce heat to low, then cover and simmer, skimming surface occasionally, for 3 - 4 hours or until meat is very tender.
For Mustard sauce: Melt butter in a saucepan over medium heat. Add flour. Cook, stirring for 1 minute or until roux is cooked. Remove from heat. Gradually stir in milk until combined. Return to heat. Cook, stirring for 3 to 4 minutes or until thickens and comes to a simmer. Remove from heat. Stir in mustard and parsley, season.
Drain beef, discarding liquid and vegetables. Thinly slice meat, then serve with seasonal roast vegetables and sauce. Garnish with parsley.
