Oil the insert of a large slow cooker. In a large bowl, toss meat with flour and salt and pepper to taste. Scatter half the onions in the slow cooker. Add half the meat. Add remaining onions and top with remaining meat. Cover and cook on low for 8-10 hours, or until meat is very tender.
In a food processor or blender, chop anchovies, garlic and parsley very fine. Add mustard and vinegar and pulse until just blended.
Skim the fat off the stew. Stir in anchovy mixture. Taste for seasoning and serve.