Place beef, 1/2 onion, garlic and bay leaves in a saucepan and cover with water. Add salt to taste. Bring to a simmer and cook 2 hours, until meat is tender. Drain and shred the beef.
Place the apple cider vinegar in a bowl and add the brown sugar; stir. Stir in tomato sauce, mustard and chilli flakes. Add salt to taste and set aside.
Heat oil in a frypan and saute onion slices until softened, about 3 minutes. Add two tablespoons of the vinegar mixture and simmer 2 minutes. Remove from frypan and set aside.
Add the brisket to the frypan and season with cayenne, pepper, paprika, garlic powder and salt. Saute 3 minutes, stirring. Pour in the rest of the vinegar mixture and simmer 5 minutes.
Cut the baguette into two, then open each half like a roll without cutting all the way through. Place the cheese in each roll and heat in the microwave to melt the cheese. Fill with the beef mixture and top with the onions.