Heat the oven to 180°C (Gas Mark 4 / 350°F).
Add the sliced red onion and peppers to a large baking dish.
Add all the other ingredients, apart from the mozzarella cheese.
Mix well to combine, making sure the rice is submerged in liquid.
Bake for around 30 minutes, then mix well. The rice should be almost cooked.
Top with grated mozzarella, and return to the oven for a further 15 minutes, or until the cheese has crisped up to your liking.
Serve with guacamole, sour cream, or any other additional toppings, if you like.
