Lightly brush each of the sourdough slices with oil, sprinkle with the salt flakes and toast under a preheated hot grill for 30 seconds–1 minute on each side until golden.
Meanwhile, heat the remaining oil in a frying pan, add the onion and cumin seeds and cook over a medium heat for 5 minutes, stirring occasionally, until golden and softened.
Add the tomatoes and cook for a further 1 minute, taking care not to let them lose their shape. Add the mozzarella and remove the pan from the heat, shaking it a little to warm the cheese very slightly.
Divide the mixture between the toasts and serve scattered with the coriander.