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- Heat one tablespoon of oil in a large frying pan over a medium-low heat. Add the onions, season with salt and pepper and cook for 25-30 minutes, stirring occasionally, until golden brown. Transfer to a plate and allow to cool.
- Return the pan to the heat, add the spinach and cook for 3-4 minutes until just wilted. Transfer to the plate with the onions and allow to cool.
- Return the pan to a medium-high heat. Add the remaining olive oil and place the mushrooms, top side down, into the pan. Cook for five minutes until golden, then turn them over and cook for a further five minutes. Transfer to a plate lined with kitchen paper to catch the liquid they release as they cool.
- Refrigerate the onions, spinach and mushrooms until chilled.
- Preheat the oven to 200˚C/180˚C fan/gas mark 6. Cover a large baking tray with parchment paper and place the pastry on top.
- Spread half of the onions lengthwise down the centre of the pastry, leaving a 2cm border at each end. Top with half of the spinach.
- Season the mushrooms well with salt and pepper and spread the mustard over their tops. Place the mushrooms on top of the spinach. Top the mushrooms with thyme and the remaining spinach and onions.
- Carefully pull the pastry over the top of the mushroom mixture to form a log, pressing to seal the edges. Flip over the log so that the seam is at the bottom.
- In a bowl, mix the egg and water together.
- Brush the pastry with the beaten egg mixture. Use the tip of a sharp knife to pierce the pastry in 3-4 places in order to allow steam to escape. Bake for 30-35 minutes until golden and flaky.
Per Serving: 561kcals, 38.1g fat (8.8g saturated), 48.1g carbs, 4.8g sugars, 9.8g protein, 5.4g fibre, 0.347g sodium
MAKE IT YOURS: If you aren’t keen on mustard, try adding sliced Brie instead!