Roll out the pastry to line a 26 cm loose bottom tart tin and bake blind for 15 minutes at 200 C. Remove the parchment and beans and bake for a further 5 minutes until golden.
Melt the butter in a pan and fry the onions over a low heat for 15 minutes then add in the sugar and vinegar. Stir over the heat until the liquid has evaporated.
Put the sweet potatoes on a baking sheet and toss in the oil, season and roast for 20 minutes.
Place the onions in a later on the pastry base, add the sweet potato and top with the goat cheese.
Bake for a further 5 minutes until the cheese starts to melt.