Preheat the oven to 160°C (325°F) mark 3. In a frying pan stir fry the onions for 1-2 minutes in a little oil. Drain the tuna well, and flake into a bowl. Cook the pasta in a pan of boiling water for about 10 minutes or until just cooked. Add the frozen peas and sweetcorn to the pan 5 minutes before the end of cooking.
To make the sauce, place milk, flour and butter in a small saucepan and whisk continuously over medium heat until the sauce begins to boil and thicken. Turn down the heat to its lowest and let the sauce cook for 2 minutes. Add the grated cheese and mustard, keeping a little cheese back for the topping. Season with a little lemon juice.
Mix the pasta with the vegetables, spring onion, tuna and cheese sauce and pour into a lightly oiled ovenproof dish. Sprinkle with the remaining cheese and bake in a preheated oven until golden on top, about 25 minutes. Serve with steamed green beans or broccoli.