In a small saucepan over low heat, bring the vegetable broth to a gentle simmer. (Tip: If using fresh corn on the cob, strip the kernels and drop the bare cobs into the simmering broth for 5 minutes to infuse extra sweet corn flavor, then discard the cobs).
In a large skillet or Dutch oven over medium heat, warm 1 tbsp of butter/olive oil. Add the zucchini and sauté for 3–4 minutes until tender and lightly golden. Transfer to a plate and set aside.
Add the remaining 1 tbsp butter/olive oil to the skillet. Add the diced onion and garlic, cooking for 2–3 minutes until softened. Stir in the Arborio rice and cook, stirring constantly, for 1–2 minutes until the edges of the grains become translucent.
Pour in the white wine (or extra broth) and stir until completely absorbed by the rice.
Begin adding the warm broth one ladleful (~½ cup) at a time, stirring frequently. Allow the liquid to absorb almost completely before adding the next ladleful. About 10 minutes into cooking, stir in the corn kernels.
Continue adding broth until the rice is creamy and tender with a subtle bite (about 18–20 minutes total). Stir the sautéed zucchini back in along with the lemon zest, lemon juice, grated Parmesan, and fresh basil.
Remove from heat, season with salt and black pepper to taste, and let sit for 2 minutes before serving.