Get a large ovenproof dish. Tip the gnocchi into it, then pour over the passata and water. Add a pinch of salt and pepper, then stir well.
Tear the basil over the top of the gnocchi. Stir again to combine.
Tear the mozzarella over the top to create a nice layer of cheese. Put in the oven for 40 minutes, until the gnocchi is soft and the cheese is golden and bubbling.