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1
Place the flour, salt and butter in a small food processor and pulse to blend the flour and butter together- it will resemble breadcrumbs.
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2
Add half of the egg and pulse again for a few seconds until it forms nuggets of dough. At this point you want to feel the dough. If it is soft, without too many dry bits then you can remove it from the processor at this point. If it still looks very dry and had not formed soft nuggets, add the remaining egg and pulse again for a few seconds, check again and if it still seems dry, add a teaspoon of ice water. If you add too much egg/water add more flour in the processor- do not try to add by hand. The less you work the pastry the more tender and pliable it will be. I found that I only needed 1/2 and egg and no water but all flour is different.
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3
Once you have a soft dough, bring it together gently into a round disk on a flat surface- ideally with cold, dry hands! Wrap in cling film and chill in the fridge for at least 30 minutes.
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4
On a lightly floured surface roll the dough out into a 10-11 inch circle. The best way to do this is to bash the dough with the rolling pin evenly in one direction, turn it a quarter turn and bash it again the other way, repeat- this helps for a more evenly rolled dough, helps keep the circular shape, stops it from getting to warm and prevents you from over-working the dough.
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5
Place on a baking tray, cover and chill for 10 minutes or until you are ready to assemble the tart.
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6
Firstly, slow roast the tomatoes by setting your oven to 110C/225F/Gas 1. Lay your sliced tomatoes cut side up on a large tray, sprinkle with sea salt, freshly ground black pepper and strew with thyme leaves. Place in the oven until the tomatoes have shriveled up and are mostly dried-up in the centre. You still want them to have a bit of juice. Ideally you would leave the tomatoes in the oven for 5-8 hours (if its a convection/fan oven it will be faster) but you can get away with 2-3 hours if you are really pressed for time- just turn the oven up a notch and keep an eye on them.
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7
Meanwhile, prepare the onions, In a large frying pan (non-stick is fine) heat the oil over a medium-low heat and toss in the onions. Cover the pan and let the onions steam, stirring occasionally until soft (about 10 minutes).
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8
Remove the cover and turn the heat up to medium-high, stir with a wooden spoon every minute or so, scraping the bottom of the pan to release any areas that are sticking. Every so often add a tablespoon or so of water to help prevent too much sticking (which will cause the onions to burn). You want to cook the onions until they have gone a lovely golden colour and appear sticky. You can speed up the process by adding a teaspoon of sugar but it will still take about 20 minutes (longer if you have more onions in the pan). Add the garlic if you are using a minute before you take the onions off the heat. Season to taste with salt and pepper.
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9
Now for the assembly and cooking. Heat the oven to 200C/400F/Gas 6. Brush your tart tin with oil. Lay the tomatoes cut side down evenly in the bottom of the pan in one layer. Top with your caramelised onions in an even layer.
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10
Beat your goats cheese until soft (if you are using ricotta you will need to season it or add some grated parmesan for extra flavour). Carefully spread the cheese over the onions as evenly as possible.
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11
Lay your pastry over the top of the tart and tuck the edges into the tart tin- if you have more than a couple of centimeters (one inch) of dough around the sides, trim it off with a pairing knife before tucking in the edges. Brush with the lightly beaten egg, using a pastry brush and bake in the oven for about 30 minutes, or until the pastry is golden and appears crispy (it should feel sandy when you touch it). Remove from the oven and allow to stand for 10 minutes before turning it out upside down on a large plate. Garnish with the basil leaves and a drizzle of olive oil.
