Place 1 tortilla on a plate and spread with the salsa. Sprinkle with the spring onions, tuna, sweetcorn and cheese. Place the second tortilla on top and press down.
Heat a large frying pan and brush with oil. Place the quesadilla in the pan and cook over a moderate heat for 2–3 minutes, pressing down with a spatula until the cheese starts to melt.
Place an inverted plate over the pan and turn the pan and plate together to tip the quesadilla on to the plate. Slide back into the pan and cook the other side for 2–3 minutes. Remove from the pan and cut into wedges.