Preheat the oven to 200C/400F/Gas 6 and preheat the grill to high.
Place the red onion, courgette and tomato in a roasting tray. Season with salt and freshly ground black pepper and drizzle over the olive oil.
Place into the oven for 8 minutes until the vegetables are soft and golden.
Remove from the oven, then place the vegetables on the slices of toast. Top with the mozzarella slices, then place under the grill for 2 minutes until golden. Serve.