Roll out the pastry, either into a square or round (or an irregular free-form shape is fine). It shouldn't be any thicker than a quarter dollar coin.
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 center has risen, gently flatten it. Turn the heat up to 200┬░C.
Meanwhile make the topping. Heat the oil in a saucepan and add the onions. Make sure they get coated with all the fat, add 2 tbsp water, season 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 catching and burning at the bottom. You may need to add a splash more water. The onions should be completely soft.
Stir in the rosemary. If the mixture is very moist - almost wet - turn the heat up to drive off the excess. 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 4 cm 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 get toasted). The cheese should be golden in patches and the pastry should be cooked but not too dark in color.
Serve immediately. If you like you can add sprigs of watercress or leaves of rocket once the tart is cooked - just strew them over the top and serve.