Preheat the oven to a high grill setting.
Put the butter and saffron into a small saucepan on a medium heat. When the butter has melted, set aside to infuse.
Place the courgettes on a parchment-lined baking tray and toss with 2 tbsp of oil, ⅓ tsp of salt and a good grind of pepper.
Arrange them cut side up and grill for 15–20 minutes, until nicely charred and softened.
Towards the last 10 minutes of grilling time, make the sauce. In a large bowl, whisk together the cornflour and water until smooth, then add the yoghurt, garlic, dried mint, the remaining oil and salt.
Whisk to combine, then transfer to a large, non-stick sauté pan on a medium heat.
Cook, stirring continuously, for about 10 minutes, or until thickened slightly and warmed through. Do not let the sauce boil, or it will split.
Transfer the warm yoghurt sauce to a plate with a lip and top with the courgettes, grilled side up.
Spoon over the saffron butter, then sprinkle with the coriander seeds and mint leaves.
Squeeze over the lemon half and serve right away.
