Preheat the oven to 190°C/gas 5. Grease a baking tray or line with a non-stick liner.
If the spinach is mature, tear out any tough stalks. Wash the spinach thoroughly, then pack it, with just the water that clings to it, into a saucepan. Cover, put over a medium heat and wilt the spinach in its own liquid – this takes only a few minutes. Drain and leave in a colander to cool. When cool enough to handle, squeeze out as much liquid as you can with your hands, then chop the spinach roughly.
Meanwhile, heat the oil in a frying pan over a medium heat. Add the onion and cook gently, stirring often, for 10–15 minutes, until really soft and golden. Add the garlic for the last few minutes. Add the thyme leaves, then the chopped spinach, and season if you like.
Roll out the puff pastry on a lightly floured surface to a square, about 5mm thick, and lift onto the baking tray. Spread the spinach mix over the pastry, leaving a margin along the edges. Scatter over the grated cheese and bake for 15–20 minutes or until the pastry is puffed and golden, and the cheese is turning golden too. Serve warm or cold.
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