Preheat the oven to 180°C.
Fry sliced onions in butter over medium-high heat for 5 minutes until colored, then cool.
Grease an ovenproof pan with butter, sprinkle with sugar, and layer the onions on top.
Cut a puff pastry disc 2cm wider than the pan diameter.
Lay the pastry over the onions and tuck the edges down the side of the pan.
Chill the pan in the fridge for one hour.
Simmer balsamic vinegar in a separate pan until reduced to a glaze.
Bake the tart for 15 minutes until the pastry is golden.
Carefully invert the tart onto a plate so the onions are facing up.
Top with goats' cheese slices and drizzle with the balsamic glaze.