Preheat the oven to fan 160C/conventional 180C/gas 4. You will need a medium casserole with a lid. Rinse the rice well in a large bowl of water, then drain.
Melt two-thirds of the butter in the casserole and sauté the onion for about 5 minutes. Add the rice, stir well, then add the herbs, whole spices and lemon zest and cook for a minute. Now pour in the boiling stock or water and mix into the rice, along with 1½ tsp sea salt and ground black pepper to taste.
Cover and bake for 25 minutes. Leave to stand for 5 minutes, then remove the lid and fork through the remaining butter until the rice is fluffy and separated.