Trim outer fat of brisket to about 1/4 in., then pat dry. Heat oil in a large frying pan over medium-high. Add brisket and brown all over, 10 to 15 minutes. Place in a 5- to 6-qt. slow-cooker.
Swirl butter into frying pan, then add onions and carrots. Cook, stirring often, until onions are lightly browned, about 10 minutes.
Stir in paprika, Worcestershire sauce, peppercorns, bay leaf, mustard, and 1/2 tsp. salt. Pour in broth, increase heat to high, and scrape up browned bits. Reduce heat and simmer 5 minutes to combine flavors.
Pour vegetables and broth over meat. Cover. Turn heat to low (meat should simmer gently but not boil) and cook until meat is tender when pierced with a fork, 6 to 8 hours. Lift meat to a platter, cover with foil, and let rest 15 minutes.
Pour sauce into a large frying pan and boil over high heat until reduced to about 1 cup. Slice brisket across the grain, top with parsley, and serve with sauce.