Preheat the oven to gas 6, 200°C, fan 180°C.
Layer up the lasagne in a baking dish, starting with a third each of the ragu, then the pasta, then the white sauce.
Repeat twice.
Top with the Parmesan and mozzarella then bake in the oven for 40-45 mins, until piping hot and crisp and bubbling on top.
Serve immediately.