- Preheat the oven to 200°C/Fan 180°C.
- Cook the rice according to the pack instructions in a pan of unsalted water, adding the sweetcorn for the last 3 minutes of the cooking time. Drain well, and transfer to a bowl.
- Divide the mackerel into small pieces and add (with the sauce) to the rice and sweetcorn, together with the tomatoes. Mix gently until well combined. Season with black pepper.
- Cut the peppers in half, cutting carefully through the stalk. Remove the seeds and pith and place on a baking tray, cut side up.
- Stuff the peppers with the rice mixture then top with the cheese.
- Place in the oven for around 30 minutes until the peppers have softened, the filling is piping hot and the cheese browned. Serve with the mixed leaves.