500g butter puff pastry
1 egg, beaten
600g peeled onions, halved, thinly sliced
2tbsp vegetable oil
1tsp chopped thyme leaves
Two handfuls of baby spinach leaves
100g goat's cheese
Roll the puff pastry to one-third of a cm thick, leave it to rest for 15 minutes then cut 4 circles about 8cm in diameter. With a slightly smaller cutter (about seven to seven and a half cms) mark it inside the edge without cutting through the pastry. Prick the centre all over with a fork to prevent the pastry rising too much, then put them on a baking tray, brush the edges with beaten egg and leave to rest in the fridge for 1 hour.
While the pastry is resting, gently cook the onions and thyme in the vegetable oil on a low heat with the lid on for 4-5 minutes until they begin to soften, stirring every so often. Season with salt and pepper, add the butter and continue cooking the onions for another 4-5 minutes with the lid on until they begin to get really soft. If they are beginning to burn, just add a little water. Remove the lid, turn the heat up a little, add the spinach and cook for a further 5 minutes.
Preheat the oven to 200C/gas mark 6. Bake the tarts for 10 minutes or until lightly coloured then remove from the oven and leave to rest for 10 minutes. With the point of a sharp knife, carefully cut around the inner edge of the pastry and push the centre down.
Spoon the onion and spinach mixture into the centres of the pastry. Arrange the goat's cheese on top and cook for another 6-7 minutes until the cheese begins to melt. Serve hot or warm.