Heat half the butter in a large saucepan over a medium heat. Add the onion and gently sauté for about 5 minutes until soft.
Add the rice to the onion, and stir until all of the rice has been coated by the butter.
Add the stock in a ladleful at a time, stirring frequently, waiting until all of the stock has been absorbed by the rice before adding another ladleful.
After about 10-15 minutes, add the courgette. Stir in well before continuing to add the stock.
Once all of the stock has been added, stir in the tomatoes. Wait until the juices from the tomatoes have absorbed before finally stirring in the spinach.
Remove from the heat and add in the remaining butter along with a large handful of freshly grated parmesan.