In the 2-quart sauce pan, bring the vegetable broth to a simmer. Keep it hot but not boiling.
In the 3-quart sauce pan, heat ¼ cup olive oil over low heat. Add the onion and sauté slowly until soft. Add the zucchini and garlic and sauté over medium heat for 2 to 3 minutes. Add the rice and stir until the rice is translucent, about 1 to 2 minutes.
Reduce temperature to low. Add the hot broth to the rice mixture one ladleful at a time, stirring frequently. 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. Continue cooking until all the broth has been added.
Test the risotto to make sure it’s al dente, but not too hard. If it’s still a little crunchy, add a ladleful of water and continue stirring. When risotto is ready, remove from heat and stir in the remaining 2 tablespoons of olive oil and the Grana Padano cheese. Serve immediately.