Make the pastry first. Put the flour, butter, cheddar and salt in a food processor. Pulse a few times, just until the butter and cheese are in smaller pieces. Measure 150ml of very cold water into a jug. Drizzle the water
into the pastry, little by little, pulsing between additions, until it comes together into a rough ball – it will feel quite doughy. Wrap in cling film and chill for 1 hr.
Meanwhile, in a pan over a low heat, very gently soften the onions in the butter and a drizzle of olive oil. Stir in the sugar, thyme and sage, turn up the heat and cook for a few mins more until golden and sticky. Season and set aside to cool a little.
Heat oven to 200C/180C fan/gas 6. Dust a big baking sheet with more flour, then roll out the pastry on it
to a rough circle, a little more than 0.5cm thick. Mix the onions with the breadcrumbs and Parmesan.
Leaving a good 5cm border all the way round, spread the onion mixture over the base, then arrange the tomatoes on top. Hand-roll and pinch the pastry edges up around the filling, pressing firmly to hold. Scatter over more thyme, plenty of seasoning and a pinch of sugar. Can now be covered and chilled for up to 12 hrs.
Drizzle with a little olive oil and bake for 25 mins until the pastry is crisp
and golden. Serve warm or at room temperature, with a big salad, if you like.