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Preheat the oven to 240°C, fan 220°C, gas 9. Put a large baking sheet on the top shelf of the oven to preheat while you make the tart. Remove the puff pastry from the fridge to soften slightly.
Mix the olive oil, maple syrup, sherry vinegar and thyme in a large bowl with a good pinch of salt and pepper. Toss in the chicory so that all leaves are well coated in the dressing. Cut the beetroot into thin wedges.
Unroll the pastry onto a large sheet of baking paper. Score a border 2cm from the edge.
Place the dressed chicory in overlapping lines within the border, then dot the beetroot wedges in any gaps, and scrape any remaining dressing and thyme leaves from the chicory bowl over the top. Use the baking paper to lift the tart onto the hot oven tray.
Cook for 15 minutes; pull the tart out and dot the blue cheese and walnuts over the top, drizzling any dry-looking bits of chicory with olive oil. Cook for 3-5 minutes more, or until the pastry is crisp. Leave to stand for a few minutes and slice into quarters to serve.