Ingredients

Step by step

1. In a large, deep-frying pan, heat the oil over a medium heat and add the butter. Add the onions, herb sprigs and some seasoning. Gently cook the onions over a medium heat for 50-60 minutes until completely soft, stirring every so often.
2. Meanwhile, in a food processor, blitz the butter, flour and salt to a crumb-like texture for the pastry. Add the thyme leaves and Gruyère, then gradually add the egg mixture and pulse to bring the dough together (you may not need all the egg). Gently knead and shape the dough into a flat disc. Wrap and chill for 30 minutes to let it rest. If you don’t have a food processor, put the butter, flour and salt into a mixing bowl and rub together with your fingertips to a crumb texture, then add the rest of the ingredients and bring together.
3. Roll the pastry out on a lightly floured surface to the thickness of a £1 coin, then transfer it to line a deep 23cm fluted tart tin, trimming off the excess. Chill in the freezer for 15 minutes.
4. Heat the oven to 200°C, fan 180°C, gas 6. Line the pastry with crumpled baking paper, then fill with baking beans or dry rice. Bake the pastry case for 20 minutes, then remove the paper and baking beans and return to the oven for 5-8 minutes until there are no uncooked patches, and the pastry feels sandy.
5. When the onions are completely soft, add the vinegar and sugar and cook for a further 10 minutes on a higher heat, stirring occasionally, until lightly caramelised.
6. Spoon the onions into the tart case (removing the herb sprigs). Beat the eggs and cream with some seasoning and pour over the onions. Scatter the cheese on top, followed by a few thyme leaves, then transfer the tart tin to a baking tray and cook in the oven for 20-25 minutes or until set.
7. Leave to stand for 5-10 minutes before serving warm, or serve at room temperature, with a classic French-style green salad dressed with vinaigrette.