Preheat oven to 180 degrees Celsius. Place the fennel and caraway seeds, garlic, salt and olive oil in a mortar and pestle and grind to combine. Rub the beef with the spice mixture and place in a large, heavy-based saucepan. Add the balsamic vinegar and brown sugar, cover with a tight-fitting lid and cook for 3-3 ½ hours or until tender, turning halfway through. Remove the beef from the cooking liquid and use 2 forks to shred. Discard any fat. Return beef to the pan and mix with the cooking liquid. Set aside.
To make the celeriac and fennel remoulade, place the sour cream, mustard, vinegar, salt and pepper in a bowl and mix well to combine. Add the celeriac and fennel and mix to coat. Fill the rolls with the remoulade and beef to serve. Makes 8.