Preheat the oven to 180C/ gas mark 4.
For the shortcrust pastry: put the flour, salt and butter in a food processor. Blend for a few seconds, add half of the beaten egg and continue blending. If necessary, add a little more egg, so there's just sufficient to bring the dough together. (If making this by hand, rub the butter into the flour and salt until it resembles coarse breadcrumbs, make a well and pour in half of the beaten egg. Use your hands to work the mixture into a dough, adding a little more egg if necessary.)
With your hands, flatten out the ball of dough till it is about 3cm thick, wrap or cover, and place in the fridge for at least half an hour (it will keep well for a couple of days in the fridge.
Take the dough out of the fridge and put the pastry between 2 sheets (bigger than your tart tin) of cling film. Using a rolling pin, roll it out until it is no thicker than 1\4cm. Make sure to keep it round, if the tin is round, and large enough to line the base and sides of the tin.
Removing just the top layer of cling film, put the pastry upside down (cling film side facing up) in clean 28 cm tart tin. Press the pastry into the edges, cling film still attached, and using your thumb 'cut' the pastry on the edge of the tin. Remove the cling film and chill the pastry in the freezer for at least 10 minutes. Reserve any leftover pastry.
Once chilled, line the pastry with greaseproof paper, fill with baking beans, or dried pulses and bake for 15-20 minutes or until the pastry feels dry to the touch.
Remove the paper and beans. If there are any little holes or cracks in the pastry, patch them up with any leftover raw pastry. Brush the tart with leftover beaten egg and return to the oven for 2 minutes. Remove from the oven and set aside.
For the tart filling: melt the butter in a medium saucepan, add the onions, cover and sweat over a gently heat until completely soft. Allow to cool.
In a bowl, whisk the eggs and yolks, add the cream, stir in the onions, tomatoes, chopped chives and mackerel. Season to taste.
. Pour the filling into the tart shell and cook in the preheated oven for about 35 minutes until just set. Allow to sit for 2 or 3 minutes before removing it from the tin. Serve either warm or at room temperature accompanied by a salad of rocket, grated beetroot, tomato and a little olive oil.
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