Peel the onion and chop it into fine brunoise, that is, into small cubes. Heat half of the butter in an ovenproof casserole and sauté the onion. When it begins to become transparent, add the herbs and rice. Sauté couple of minutes so that the rice is well impregnated with butter.
Next we wet the rice with boiling water and season to taste. If we use white broth and it already has salt, it is not necessary to add more. We stir slightly and cook over high heat for 6 or 7 minutes. Then we transfer the casserole to the oven, preheated to 160 ºC, where to cook for 12 more minutes.
Finally, we remove the casserole from the oven and immediately add the rest of the butter. We stir so that it integrates well and leave rest for a few moments before serving. We can serve it molded or well distributed in a source.