Drain Greek yoghurt in a fine sieve overnight. Mix the drained yoghurt with crushed garlic clove and a pinch of salt.
Heat oven to 180c/fan 160c/gas 4. Halve 12 plum tomatoes lengthways and put cut-side up in a tin. mix 4 tbsp olive oil, with 2 tsp balsamic vinegar, ½ tsp harissa. Pour over the tomatoes. Sprinkle with golden caster sugar and cook for 45 minutes.
Toast 4 slices of sourdough bread. spread the yoghurt mixture onto the bread, then top with the tomatoes and drizzle over any juices from the pan. Scatter with a handful of chopped coriander, drizzle with extra-virgin olive oil and serve.