Caramelised onion and thyme tart

Method

1. Melt the butter in a large saucepan, then add the onions and stir to coat. Season, then cover and cook over a very low heat for 1 hour, stirring occasionally.
2. Preheat the oven to 180°C/fan160°C/gas 4.
3. When the onions are very soft, add the white wine vinegar, sugar and mustard.
4. Cook, uncovered, for 20 minutes more or until the vinegar has evaporated. Stir through the thyme leaves, then season. Spoon the onion mixture into a 16cm blind-baked pastry case, then scatter over the parmesan and a few fresh thyme sprigs.
5. Bake in the oven for 10-15 minutes until golden. Leave the tart to cool for 30 minutes, then serve.