Cut the green of the spring onions into 5mm lengths and finely slice the remaining white onion. Stir fry the spring onions for 1-2 minutes in a little oil. Drain the tuna well and flake into a bowl. Grate the cheese for the sauce.
Cook the peas in boiling water until tender, for about 5 minutes. In a large saucepan of boiling salted water, cook the pasta until al dente.
To make the sauce, melt the margarine, add the flour and cook until sanding in colour and texture. Add the milk, whisking all the time and when it's smooth and has thickened, add the cheese, keeping a little back for the topping. Stir in the cayenne pepper.
Mix the pasta, peas, spring onions, tuna and cheese sauce into a deep dish, sprinkle with the remaining grated cheese and bake in the preheated oven until golden on top, about 25 minutes.