Heat the oven to 200°C/400°F/Gas mark 6. Grease two baking sheets.
Heat the olive oil in a large frying pan and fry the onions over a low heat for 20 minutes, until softened. Add the honey, soy, thyme and pepper and stir for a further 10 minutes on low. Add 1 tablespoon of cold water to the hot pan and stir, mixing in all of the juices from the base of the pan. Take off the heat and leave to cool.
Slice the pastry sheet in half lengthways and put one piece on each baking sheet. Lightly score a line 5cm/2in in from the edge around each sheet to create a border. Divide the onion mixture evenly between the pastry shets and spread within the border, to allow the pastry to puff up around the edge. Lay the goat's cheese slices at intervals over both pastries and bake for 25 minutes, until cooked and golden.