Heat a little of the duck fat in a heavy frying pan and slowly cook the sausages for about 10 minutes. Remove from the pan and set aside. Meanwhile, peel and slice the onion.
Now add the rest of the duck fat to the pan, add the onion slices and cook very gently until caramelised (about 10 minutes). Stir in the balsamic vinegar, mustard and honey, then add back the sausages for about 5 minutes, to warm up. Pile into the ciabatta with the sauce of your choice.