Boil the kettle. Cut the lasagne sheets lengthways into ½cm strips. Finely slice the bacon, then the mushrooms, keeping them separate. Pick and chop the rosemary leaves. Finely grate the Parmesan into a small bowl, then beat in the egg.Put a 28cm frying pan on a high heat. Once hot, put a little drizzle of olive oil into the pan with the bacon, rosemary and a generous pinch of black pepper. When lightly golden, add the mushrooms. Cook for 2 minutes, stirring regularly, then scatter the pasta into the pan. 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, stirring regularly. Turn the heat off, let it sit for just 30 seconds, then stir in the egg mixture, shaking and stirring vigorously until you have a delicate, silky sauce – you need it to be off the heat so you don’t scramble the egg, but equally you need to keep it moving to get a smooth sauce. Season to perfection, and finish with a kiss of extra virgin olive oil and an extra grating of Parmesan, if you like.