Preheat the oven to 180°C. Heat the olive oil in a large frying pan over a moderate heat and saute the spinach in batches for 1-2 minutes, stirring occasionally. Transfer to a colander to drain. Meanwhile, blanch the lasagne sheets in a large saucepan of boiling salted water for 1 minute, then drain and refresh in iced water.
Break the ricotta down in a small mixing bowl and season. Spread a little into the base of a 30cm x 15cm x 8cm roasting tray. Layer the lasagne sheets on top of the ricotta then top with a mixture of the flaked tuna and the spinach. Spoon a little more ricotta on top and then layer with more lasagne sheets.
Continue in this fashion until you have a final layer of lasagne on top and have used up all the ricotta and spinach, making sure you season well between layers. Sprinkle the mozzarella on top in an even layer and bake in the oven for 20-25 minutes until golden and bubbling. Remove and serve immediately.