Preheat the oven to 180ºC.
Heat the oil in a frying pan over and sauté the onion for about 10 minutes, until softened. Add the balsamic vinegar, stir for 30 seconds, then set aside to cool.
In a mixing bow, combine the sausage meat with the egg, sage, breadcrumbs, preferably with your hand. Then add the remaining ingredients and the balsamic onion, then combine well.
Transfer the mixture into a medium loaf tin. Cover with foil and bake for 30 minutes.
Uncover the tin and bake for another 15 minutes or until the top is golden brown.
Turn the loaf onto a plate and let it cool a bit before slicing it. (You can decorate it, if preferred)
Serve warm with roasted poultry or cold with salad and chutney.