Put the oil in a saucepan over a low heat. Add the onion and red pepper and cook gently for about 5 minutes until softened. Add the garlic and cook for another 30 seconds. Stir in the rice, then pour over the stock and season with salt and pepper.
Bring to the boil, then reduce the heat and simmer, stirring occasionally, for 10–15 minutes until all the stock has been absorbed and the rice is tender.
Stir in the sweetcorn and tuna, and cook briefly over a low heat to heat through. Serve immediately garnished with the spring onions.