Finely mince the onion and cut the zucchini in small cubes. Drizzle a large pan with olive oil, then sauté the onion and zucchini for a few minutes, until they start to brown. Season lightly with salt and black pepper.
Add the Carnaroli rice to the pan and cook for a couple of minutes.
Add a few ladles of vegetable stock, until the rice is covered, and turn down the heat.
Let the rice simmer, mixing it once in a while to make sure it doesn't stick. When the stock is almost completely absorbed, add some more. Continue until the rice is al dente.
About 5 minutes before it's done, add the cream or milk. In the end, the rice should be creamy and not soupy. If there's too much liquid, turn the heat to high and let it evaporate.
When the rice is cooked, turn off the heat and add the butter and grated Parmigiano cheese. Mix well until the butter is fully melted. Serve immediately.