Preheat the oven to 180°C, fan 160°C, gas mark 4. Prepare the peppers by slicing in half lengthways and removing the seeds and core, keeping the stem attached.
Heat 1 tablespoon olive oil in a shallow frying pan and fry the onion until soft. Add the drained kidney beans to the pan and heat through. Add the rice and hot stock. Simmer for 15 minutes.
Place the peppers on a baking tray, then fill with the rice mixture and top with the mozzarella. Sprinkle any leftover rice onto the baking tray. Bake in the oven for 20 minutes.
Serve hot with a salad garnish of the sliced tomatoes and wild rocket leaves.