Preheat your oven to 190°C.
Mist a frying pan with low calorie cooking spray and fry the red onion until it starts to brown then add the balsamic. Turn off the heat, but leave the pan on the stove.
Mash the sausages in a bowl, add the sage, cooked red onion and egg. Mix well until all combined.
Empty mix into a small foil tray making sure to squish it down. Place in the oven and bake for 35-40 minutes until golden on top.