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First, make the salt crust. Blitz the rosemary and salt in a food processor, then add the flour and egg whites with 200ml water. Blitz again to bring together. Tip the mixture onto a clean surface and knead for 5 mins to form a smooth ball, then wrap in cling film and chill for 1 hr.
Heat oven to 160C/140C fan/ gas 3. Roll the dough out to the thickness of a £1 coin. Put the beetroots in the middle and draw the edges up to form a bag. Bake on a tray for 2 1/2 hrs
Meanwhile, make the pickling liquor for the onions. In a saucepan, combine the vinegar, sugar, mustard seeds and 1 tsp salt over a medium heat, and bring to the boil. Add the dill and leave to cool for 30 mins. Strain the mixture onto the onions, and leave for a further 1 hr, then drain. In a small bowl, mix the dressing ingredients with some seasoning and set aside.
Remove the beetroots from the oven and leave in the salt crust until cool enough to handle. Open the salt crust with your hands and discard. Peel the beetroot, cut into wedges and drizzle with a little rapeseed oil.
Mix the beetroots, onions and beetroot leaves in a large mixing bowl, then add the dressing. Reserving a little of each for decoration, add the chives, apple and hazelnuts, toss well, then tip into a large serving bowl. Crumble the feta over the top with the reserved ingredients, then let everyone help themselves. Best served slightly warm.