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.
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.
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.
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.
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.