Heat the ¼ cup olive oil in a heavy-bottomed 2-quart saucepan over low heat. Add the onion and sauté slowly until soft. Add the zucchini and garlic, and briefly sauté over moderate heat, 2 to 3 minutes. Add the rice and stir until the rice is translucent, 1 to 2 minutes.

Add the hot broth to the rice mixture one ladleful at a time, stirring frequently. The rice should cook very slowly over low heat. Wait until the liquid is absorbed before adding the next ladleful of broth. Midway during cooking, add the basil and salt and pepper to taste.

When the risotto is al dente, Add the remaining 2 tablespoons olive oil and the cheese. Turn off the heat and stir briefly to combine. Serve immediately.