Preheat the oven to 200°C fan.
Place the tomatoes in a mixing bowl with the olive oil, cumin seeds, sugar, garlic, thyme, oregano sprigs, lemon strips, half a teaspoon of flaked salt and a good grind of pepper. Mix to combine, then transfer to a baking tray just large enough to fit all the tomatoes together snugly. Roast for 20 minutes, until the tomatoes are beginning to blister and the liquid is bubbling. Turn the oven to the grill setting and grill for six to eight minutes, until the tomatoes start to blacken on top.
While the tomatoes are roasting, combine the yoghurt with the grated lemon zest and a quarter of a teaspoon of flaked salt. Keep in the fridge until ready to serve.
Once the tomatoes are ready, spread the chilled yoghurt on a platter (with a lip) or in a wide, shallow bowl, creating a dip in it with the back of a spoon. Spoon over the hot tomatoes, along with their juices, lemon skin, garlic and herbs, and finish with the picked oregano and chilli flakes. Serve at once, with some bread.