Trim away the root ends and the dark green leaves from the spring onions, then finely slice and set aside.
Fill a large saucepan to three-quarters full with water and bring to the boil. Add the pasta and cook according to packet instructions until al dente (tender to the bite), then carefully drain using a colander.
Put the pasta back into the saucepan and add the spring onions, tuna, sweetcorn, and mayonnaise, and mix everything together. Easy!