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Heat the oven to 200C/180C fan/gas 6. Tip the celeriac wedges into a large bowl with the olive oil, paprika, chilli flakes, maple syrup and most of the thyme leaves. Season and toss well to evenly coat. Tip onto a baking-parchment-lined baking tray, spread out into a single layer and roast for 30-35 mins until tender when pressed with the tip of a sharp knife. Turn halfway through cooking, brushing with any glaze on the tray. Leave until cool enough to handle.
Meanwhile, fry the shallots in a drizzle of oil in a frying pan set over a medium heat with a pinch of salt until soft, about 5-6 mins. Cool.
Mash the soft cheese, herbs, a pinch of chilli flakes, a few more thyme leaves and some seasoning together in a bowl. Fold in the shallots. Place a large baking sheet in the oven to heat.
Roll the pastry out on a floured work surface into a large rectangle around 38 x 28cm and 3mm thick (it should be around the same size as your baking sheet). Transfer to a large sheet of baking parchment.
Spread the soft cheese mixture over the middle of the pastry, leaving a 5cm border. Arrange the celeriac wedges on top in a single layer. Carefully fold the exposed border over the edge of the filling so it slightly overlaps the celeriac wedges.
Brush the border with the milk and, using the baking parchment to help you, carefully transfer the tart to the hot baking sheet. Bake for 35-40 mins until the pastry is cooked through (gently lift it with a fish slice to check the bottom is cooked) and the celeriac is golden and caramelised. Scatter with more chives, the remaining thyme and a pinch more chilli flakes (if you like) to serve.