Tomat- och karamelliserad löktart tatin

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1. 1
   Heat the butter and oil in a frying pan until the butter has melted. Cook the onion over a medium heat for about 10 minutes until beautifully golden, stirring often. Stir in the sugar and cook for another couple of minutes. Tip the onion and juices into a bowl and set aside. Preheat the oven to 200C/gas 6/fan 180C.
2. 2
   For the pastry, rub the butter into the flour to make fine crumbs. Stir in the parmesan, thyme and a pinch of salt. Add the egg yolk and 2 tbsp cold water, then mix to make a dough. Wrap in cling film.
3. 3
   For the tomatoes, heat the butter and oil in a 20cm tart tatin tin on the hob until quite hot. Stir in the sugar and garlic, then put in the plum tomatoes cut-side down with a few thyme sprigs, and sizzle for no more than 1 minute. Scatter in the whole cherry tomatoes and tuck in a bit more thyme. Take off the heat and spread the onions on top. Season with salt and pepper.
4. 4
   Roll out the pastry until it's slightly bigger than the top of the tin. Lay it over the onions and tuck any excess down the sides.
5. 5
   Bake on a baking sheet for 25 minutes until golden. Cool for 5 minutes, invert a plate over the top and upturn the tart onto it. Scatter with extra thyme and black pepper.