Begin by preparing the onions, which should have skins removed and be cut in half from root to stem. Then place a frying pan with the butter over a medium heat and as soon as the butter begins to sizzle, quickly add in the thyme and arrange the onions with their cut side down.
Add salt and pepper to the top of the onions and sprinkle the vinegar over them also. Now turn the heat down on the pan and leave them simmer for 10 minutes.
After the 10 minutes cover with foil and place in an oven preheated to 180°C for 40-50 mins.
Once the onions are cooked you can cut out your pastry circle, (the pastry circle should be the size of the pan you are working with) and place over the onions. Take care to tuck in the edges around the onions like a blanket.
Place the tarte back in the oven for approx 20-25 minutes or until golden brown. Leave to cool for 5 minutes and then turn out onto a plate. Scatter some Goats cheese over the top if serving immediately or if you make it in advance, it can be reheated in a 150°C oven in around 20 minutes.
Scatter the goats cheese just before going to the table.