Boil the kettle. Cut the lasagne sheets lengthways into 1cm strips to make tagliatelle. Peel and finely chop the onion. Finely slice the chilli. Roughly chop the top leafy half of the parsley, then finely slice the stalks, keeping them separate. Finely grate the Parmesan. Put a 28cm frying pan on a high heat.
Once hot, put a little drizzle of oil from the tuna jar into the pan with the onion, chilli and parsley stalks, followed a minute later by the frozen sweetcorn. Toss over the heat and once the onion is lightly golden, flake in the tuna. Scatter the pasta into the pan with the parsley leaves, then carefully pour in enough boiling kettle water to just cover the pasta – about 300ml. Let it bubble away for 4 minutes, or until the pasta has absorbed most of the water and you’ve got a nice sauce, stirring regularly and loosening with an extra splash of water, if needed. Turn the heat off, stir in the Parmesan, squeeze in the lemon juice, then season to perfection. Finish with a kiss of extra virgin olive oil, if you like.