Cook the pasta in boiling water to pack instructions, then drain well. Preheat the oven to 180ºC, fan 160ºC, gas 4.
Meanwhile, heat the oil in a frying pan over a medium heat and cook the onion gently for 5-10 minutes, until soft. Add the mushrooms and cook for a further 5 minutes until lightly golden. Add the tomatoes and juice, and break up the tomatoes with a fork.
Mix together the pasta, tomato mixture, flaked tuna and spinach. Tip into a 2-litre ovenproof dish.
Scatter over the mozzarella and bake in the oven for 20 minutes, until heated through.