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Preheat the oven to 190°C/375°F/Gas Mark 5.
Roll out the pastry into a square that will fit on your baking tray. It shouldn't be any thicker than a 10 pence piece.
Put the pastry onto a floured baking sheet and prick it all over with a fork. Then put into the oven for 20 minutes. Remove from the oven and, if the centre has risen, gently flatten it. Turn the heat up to 200°C/400°F/Gas Mark 6.
Meanwhile, make the topping. Heat the oil in a saucepan and add the onions. Coat well with the oil, add 2 tablespoons of water, season with salt and pepper and cover the pan. Turn the heat down very low and let the onions sweat for about 20 minutes. You need to check every so often to make sure they're not burning at the bottom. You may need to add a splash more water. The onions should be completely soft when ready.
Stir in the rosemary. If the mixture is very moist - almost wet - turn the heat up to remove the excess moisture. You don't want too wet a mix to go on top of the tart.
Top the pastry with the onions, leaving a rim of about 4cm round the edge, then add the beetroot wedges, then the crumbled cheese. Season with salt and freshly ground pepper and drizzle with extra virgin olive oil.
Put this back into the oven and cook for 10-15 minutes, scattering the hazelnuts over the top 3 minutes before the end of cooking time (they just need to be toasted). The cheese should be golden in patches and the pastry should be cooked but not too dark in colour.