Pre heat the oven to Gas 6, 200°C, fan 180°C.
Sieve the flour into a large mixing bowl and season with salt and pepper. Cut the margarine into cubes and add to the bowl. Rub in the margarine until the mix resembles fine breadcrumbs. Make a well in the centre, pour in a little of the cold water and start to bring the dough together with the blade of a knife. You may not need all the water to form the dough, so add the remaining water gradually. Once the dough forms a ball, wrap it in cling film and rest it in the fridge for 30 minutes.
Put the butter, brown sugar and balsamic vinegar into a flan tin which does not have a loose bottom. Place the tin in the oven to allow the ingredients to melt together and caramelise slightly. Carefully remove the tin from the oven and arrange the halved onions in the caramel with the cut sides down so that the rings of the onion will form a pattern when the tart is turned out.
Place the small chilli tomatoes in the gaps between the onions and put the tin back into the oven for a further 20 to 30 minutes, until the onions have softened.
Roll out the pastry and cut a circle which will fit over the onions; reserve in the fridge until the onions are softened.
Remove the tin from the oven and carefully place the pastry over the onions. Return to the oven for around 20 minutes, until the pastry is cooked through and golden.
When the tart is cooked, remove from the oven and allow it to cool a little before turning upside down on to a large plate so that the onions are on top. This can be decorated with goat’s cheese and grilled to finish. Serve with a green salad dressed with olive oil and balsamic vinegar.
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