1In a small saucepan, bring the vegetable broth to a simmer over medium heat and keep warm.
2Meanwhile, heat the oil in a large, wide saucepan over medium heat until shimmering. Add the onion and garlic and season with salt and pepper. Cook, stirring with a wooden spoon, until the onion has softened, about 5 minutes.
3Add the rice and cook, stirring occasionally, until the kernels start to crackle, about 1 to 2 minutes. Pour a ladleful of broth over the rice and let simmer, stirring constantly, until the rice absorbs the broth and the bottom of the pot is nearly dry. Continue adding broth a little at a time, seasoning occasionally and stirring constantly, until the rice is nearly al dente, about 20 minutes.
4Add the zucchini and continue to cook until the rice is tender on the outside but still has some resistance when you bite into it, about 5 minutes more. (You might not use all of the broth.) Stir in the Parmesan, taste, and season as desired. Serve with additional Parmesan.