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- Heat one tablespoon of the olive oil in a pan over a medium heat and cook the onions for 15 minutes or until very soft, stirring occasionally and adding a splash of water if they begin to stick. Set aside.
- Squeeze the lemon half into a bowl of cold water. Peel the celeriac and slice very thinly, placing the slices in the bowl of water.
- Preheat the oven to 200˚C/180˚C fan/gas mark 6. Line a large, rimmed baking tray with parchment paper.
- Roll out the puff pastry on a lightly floured surface into a rectangle around 20 x 30cm. Transfer to the prepared tray. Use a small, sharp knife to score a border 1cm in from the edge of the pastry, being careful not to cut all the way through. Pierce the pastry inside the border all over with a fork. Bake the pastry for 10 minutes until lightly golden. Remove from the oven and use the back of a spoon to press down the pastry inside the border.
- In a bowl, combine the mascarpone and mustard. Spread the mixture evenly over the pastry inside the border. Brush the border with the beaten egg.
- Dry the celeriac slices very well with kitchen paper. Pour out the lemon water from the bowl and wipe it dry, then return the celeriac slices to the bowl and drizzle over the oil. Season with salt and pepper and toss to coat.
- Scatter the Gruyère over the tart, then top with the fried onions, the rosemary and the celeriac. Add an extra crack of black pepper.
- Bake for 15-20 minutes, then serve the tart warm or at room temperature with some mixed leaves.
Per serving: 716kcals, 53.8g fat (18g saturated), 45.2g carbs (3.5g sugars), 12.1g protein, 3.7g fibre, 0.351g sodium
Top tip: Using a mandoline or a vegetable peeler will make it easier to slice the celeriac very thinly.
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