Heat a dash of oil in a frying pan, and add the diced onion, garlic, mushrooms and spinach. Cook for 5-10 minutes over a medium heat, stirring regularly, until the vegetables are fairly soft.

Add the chopped tomatoes, rice, and vegetable stock, and mix well. Transfer the mixture to a baking dish (mine measured approximately 8 x 8 inches), and add a lid (you can use kitchen foil if you don't have a lid).

Bake at 190°C (Gas Mark 5 / 375°F) for around 30 minutes, or until the rice is almost entirely cooked. Give it a stir, then sprinkle with grated cheese. Return to the oven (uncovered) until the cheese has crisped up to your liking (another 10-15 minutes).